Guide:Getting started with Hardmode

Hardmode is a setting that is switched on in a world when a player defeats the Wall of Flesh. It changes many things about the world and adds more progression options for the game. This guide is for players who have never played in a Hardmode world before. The change is permanent (Unless using third-party tools, ignored in this guide) and going into Hardmode with insufficient preparation can be at best very challenging and, at worst, deadly (And by deadly we mean dying VERY often), not counting the long-term effects over the world.

Preparations
"One does not simply walk into Mordor" or in this case, the Hardmode; one must prepare themselves and the world for it first. The main world preparations one should make are for the spread of the Hallow and Corruption or Crimson. Because they spread in a similar (though not entirely identical) way, this guide will refer to them collectively as "infection".

The primary danger caused by the spread of these infections is the destruction of already existing Environments. Any naturally occurring deserts are particularly susceptible to infection, as are the ocean sands. This means that sand, as a resource, may become uncommon or even rare, at least in their pure form, and can make Waterleaf almost unobtainable. This can be temporarily reversed with Vile or Purification Powder, which can turn respectively Pearlsand and Ebonsand into normal Sand form. The infection will spread back over the restored sand unless it is isolated or harvested.

The Oceans are also susceptible to infection. Pearlsand and Ebonsand do not permit the growth of Coral, and they cause Ocean biomes to stop producing their normal slate of enemies.

The Jungle, and the Underground Jungle are completely immune to the Hallow, but not to Corruption. Corruption can transform Jungle grass and mud into Corrupt grass and dirt, which itself can continue to spread. This process can lead to the complete destruction of the Jungle environment.

The guide explaining how to contain the infections is useful, but a full quarantine may not be entirely practical. The reason for this is that every time you break an altar to bless this world with Hardmode ores (Cobalt/Palladium, Mythril/Orichalcum, and Adamantite/Titanium), there will be a random chance that a block of stone somewhere will become Pearlstone/Ebonstone, and thus start a new region of Hallow/Corruption. This can break quarantine. Fortunately, if you don't mind waiting, you can get hardmode ores from fishing up crates and opening them in hardmode.

Preserving access
The main issue with the destruction of natural biomes is that certain harvested materials may become lost forever to this world. Several threatened biomes have unique drops found nowhere else.

An obvious way to deal with this is to simply acquire all of these unique items you need before triggering Hardmode. The items of note (and the total numbers you may need for all of its crafting possibilities are):

You could also simply have another world that you preserve in its pre-Hardmode state for harvesting. However, if you want to preserve continued access to items in this world, you will need to take certain steps.
 * Jungle Spores (62 total)
 * Nature's Gift (2, one for making a Mana Flower): Found randomly in the Underground Jungle.
 * Vine (5): Drop from Maneaters in the Underground Jungle.
 * Stinger (42): Drop from Hornets in the Underground Jungle. You may wish to get more for making Thorns Potions, Flasks of Poison, and Abeeminations.
 * Diving Helmet: Rare drop from Sharks.
 * Shark Fin (5 for Megashark): Drop from Sharks in the Ocean. You may wish to get more for making Hunter Potions or Water Walking Potions.

As many of these items come from the Jungle, protecting the Jungle will ensure that they are always available. The Hallow cannot infect the Jungle, but the Corruption can. This means that it is possible to deliberately place patches of Hallow between the Jungle and the Corruption. Given time, this will act as an effective shield against the Corruption.

However, as previously mentioned, random squares of Hallow or Corruption will appear anywhere on the map when a Demon Altar is broken (see below). Thus, it remains possible for the Corruption to breach this quarantine and infect the Jungle directly.

Better biomes and gardening
Artificial biomes and gardens are the most reliable way to maintain access to various items. Ideally, any artificial biomes or gardens should be constructed after activating Hardmode and blessing the world with ores. This will allow for more effective quarantine procedures.

When building such biomes or gardens, proper quarantine is still needed. Ensure that there are at least 3 uninfectable blocks between your biome and the outside area. Empty space can be used, so long as the Corruption's thorny vines are not a problem. If they are, then any non-infectious block will work as a barrier. (Snow blocks are ideal)

Jungle Grass Seeds, a rare drop from cut Jungle Grass, can be used to build an artificial Jungle biome. They grow on mud blocks. When constructed underground, below depth 0, this becomes an Underground Jungle biome. Once the grass spreads far enough on mud to form a biome, then the native biome enemies can spawn. To gain access to Jungle Spores, you will need to place the Jungle Grass in the Cavern Layer. This will also spawn Underground Jungle creatures.

Moonglow requires Jungle Grass (or a Clay Pot, which does not require any particular biome) to grow in. Thus, it too is endangered by threats to the Jungle. Moonglow Seeds can be valuable for building an appropriate garden. When placed underground, Moonglow gardens built directly on jungle grass will also create an Underground Jungle biome.

Desert biomes are constructed from a single, one-block-deep strip of sand. However, Antlions spawn better with 3-block deep strips. Sand can be gathered pre-infection or transformed back to its normal state via Powders sold by the Dryad.

An artificial Ocean biome can be constructed. These are created by putting a large body of water over a large body of sand near the border of the world. The background is a good way to check for distance; if it has the ocean background, then any body of water over regular sand will count.

The Corruption (but not the Hallow) can damage or destroy naturally occurring Glowing Mushroom areas. A properly quarantined Mushroom farm will allow access to Glowing Mushrooms no matter what. It does not take a very large farm to produce sufficient mushrooms for your potion needs, and they grow automatically.

Pre-Hardmode gear checklist
This is a list of what you ought to have access to before Hardmode, as well as why.

This is just a bare minimum list of things you will need. Feel free to add to this list to suit your tastes and playstyle.
 * A full complement of Health and Mana. That's 400 HP and 200 Mana (the latter without bonuses from armor or accessories). You don't want to get downright mauled by Hardmode enemies to search for Heart Crystals. Very optional, but the player can keep a couple of stacks of Honeyfins instead of Healing Potions, due to them having somewhat higher base HP-restoration, at least until a player can get a Pixie Dust farm going for a steady supply of Greater Healing Potions.
 * A full set of end-tier pre-Hardmode armor. This means Molten Armor for melee, Necro Armor for ranged, Jungle Armor/Meteor Armor for magical, or Bee armor for summon weapons . If your dodging skills are quite good, the other armors can be used along with ranged or magical weapons. Even so, you are strongly recommended to have a set of Molten Armor available to swap into, as Hardmode enemies do a lot of damage. Using Gold Armor or below is practically asking to be killed.
 * Molten Pickaxe or Reaver Shark: These are the only pre-Hardmode pickaxes that can mine cobalt and palladium, the two possibilities for the first Hardmode ore.
 * Night's Edge : The most powerful pre-Hardmode swords, for melee characters. If you do not feel confident in your dodging skills, either sword coupled with Molten Armor is a safe, reliable solution. How much you reforge it is up to you, since you will be abandoning it fairly soon in Hardmode. If the Breaker Blade drops from the Wall of Flesh, you may use it instead of these, trading damage for its size and knockback. Getting this before hard mode is advised because it is significantly harder to get in hard mode, and is used to make the Terra Blade.
 * The Sunfury and/or the Flamarang: These are "ranged" weapons that are considered melee, so they will gain the damage bonus from Molten Armor. They're good tools for keeping Hardmode monsters away. How much you Reforge it is up to you, as again, you will be getting an upgrade fairly soon.
 * Phaseblade, the color of your choice: This is not to be used directly (unless preferred.) It is here solely to craft a Phasesaber. The Phasesaber is a very powerful and very easily obtained Hardmode melee weapon, and it will save your life many times over in your Hardmode play. A Phaseblade is a component in its construction, so there is no need to reforge it. If you play on the mobile version, Arch Demons, found in the Underworld, will drop Crystal Shards, which 50 of can be used to make a Phasesaber pre-Hardmode.
 * Molten Hamaxe: While you will get a Pwnhammer from the Wall of Flesh, it only acts as a hammer. So you'll still need an axe to start off Hardmode. And really, the only thing the Pwnhammer can do that the Molten Hamaxe cannot is destroying Demon Altars. And you'll stop doing that at some point. Also, it takes less inventory space to carry one Hamaxe than a Hammer and an Axe. As well as that, getting the light modifier on it should let you by pass the chainsaws.
 * Cobalt or Obsidian Shield: Knockback can easily get you killed in Hardmode, and avoiding that can really benefit you. Consider having it reforged at the Goblin Tinkerer for defense or offense bonuses.
 * Movement gear. Getting around will be vital in surviving Hardmode, especially with low-defense armors. Also remember that every accessory can be reforged to give additional movement speed (Up to +4% per accessory).
 * Cloud in a Balloon, crafted from the Cloud in a Bottle (found in golden chests in the Underground and Cavern layers.) and Shiny Red Balloon (found in Skyware Chests on Floating Islands) at the Tinkerer's Workshop is very useful for overall streamlining of your experience. The Sandstorm in a Bottle variant may be preferred, as it has the highest in terms of jump boost. Sandstorm in a Bottle is found in Pyramids.
 * Blue Horseshoe Balloon is a direct upgrade of the Cloud in a Balloon, and repels any fall damage. It only requires a Cloud in a Balloon and a Lucky Horseshoe.
 * Grappling Hook is one of the most important tools as it has countless uses. Moreover, it is cheap (Only 2 gold coins, or a Hook (crafting material) and some Iron). If you can, upgrade your Grappling Hook to an Ivy Whip. You can however, provided you have a surplus of them, use diamonds to craft a Diamond Hook, or if you got lucky during the Halloween season, a Bat Hook.
 * Frostspark Boots, crafted from Hermes Boots (found in random underground chests), Rocket Boots (purchased from the Goblin Tinkerer), Ice Skates (found in ice biome chests), Aglet, also found randomly, and Anklet of the Wind, in the Jungle Shrines.
 * Magic Mirror or Ice Mirror: These will be useful for getting out of tight spots.
 * Optionally, you could hold onto that and obtain the rest of the items to make the Cellphone for added information which might help you towards finding enemies and ores in Hardmode. However, it is more time consuming.
 * Mana Flower, crafted from Nature's Gift and Mana Potion: If you want to consider using Magic Weapons as anything more than a backup for long-range fighting, especially for boss fights, you'll need this.
 * A ready supply of Sand Blocks: This will be used for constructing deserts, and to make glass for bottles for later potion making.
 * A supply of any seeds that you wish to use for post-Hardmode farms and biomes.
 * Diving Gear is optional, it is a useful item if you find water quite annoying in caves and underground. It is useful before and in hardmode. The Arctic Diving Gear (an upgrade of the Jellyfish Diving Gear, crafted by combining a Jelly Diving Gear with Ice Skates) has all of the benefits of both previous diving gears, while also providing extra mobility on ice.
 * Having a Dark Lance and/or Flamelash are also suggested for exploration of caves as the Dark Lance emits light, and the Flamelash's sparks bleed through blocks.
 * A strong ranged weapon for people who prefer ranged weapons, like a Molten Fury, Minishark, Phoenix Blaster, or a Star Cannon can also prove useful.

Final preparations
Once you feel ready to initiate Hardmode, there are a couple of things you should do. Your world spawned with a number of naturally-occurring Corruption or Crimson sites. Go there and find all of the Demon Altars. You cannot do anything with them; the point of this exercise is to familiarize yourself with their locations so you can reach them quickly.

You may even want to dig a back-door into the lower Chasms of Corruption, to make it easier to get down to the Altars. Hardmode Corruption or Crimson enemies are not to be underestimated.

Also, find a Demon Altar that is near your spawn or your base. Once in Hardmode, you will want to destroy Demon Altars, but you need to keep at least one around for crafting purposes.

You may also wish to build a Hellevator. Once you get some Hardmode gear, you will likely want to farm the Wall of Flesh in order to get certain items and/or money. And the Cavern layer will be very dangerous for you initially, so having a quick way to go from the relatively safe surface to the relatively safe Underworld is a good idea.

Initiate Hardmode
Kill the Wall of Flesh. For more detailed information, see this guide.

Bless the world
Once you have killed the Wall of Flesh, loot the items from the box it drops and immediately teleport back to your base with the Magic Mirror. However, you may want to farm it for powerful items it drops. The Clockwork Assault Rifle and the Laser Rifle are by far the most useful, dealing high damage at a range, while the Emblems increase damage of a type. They can be combined to make an Avenger Emblem which gives an increase to all types of damage and is used in endgame gear for all damage types. (As of 1.3, you can no longer create an Avenger Emblem with all of the emblems, but instead with one emblem and each of the Hardmode boss souls)

Your next task, after storing any acquired gear, is to take your newly minted Pwnhammer and break some Demon Altars. How many is up to you, but the more you break, the more you pollute the world. Even smashing three altars will give you plenty of ore, you'll just have to look for it longer.

Note that a number of Wraiths (usually 1-3) may spawn when you destroy a Demon Altar. You can become quickly overwhelmed if you destroy too many at once.

Once you have blessed the world to your taste (remembering to keep that one Demon Altar you identified earlier intact), return to your base.

How about not blessing the world?
Since 1.2, you can obtain hardmode ores from crates you got via fishing. If you don't want to ruin your world by random patches of corruption/crimson, you don't have to smash any altars at all. However, getting enough ore this way is much more time consuming, but it's also safer and gets you much more money (you can sell other things you get from the crates and they usually contain lots of coins as well). This also only works in the PC Version. If you choose this method, you need a large number of crates.

Gearing up in Hardmode
Treat Hardmode like you've just started to play the game from scratch. The Cavern will have a lot of enemies that are very dangerous for you, and the Corruption and Hallow should be approached with caution. Even regular Forests can kill you at night if you're not on your toes.

Your first order of business is to get a Cobalt Drill or Cobalt Pickaxe (alternatively Palladium Drill or Palladium Pickaxe), as this is the first item that can mine Mythril Ore (or Orichalcum Ore). Go into caves that you have explored before and search for newly lain patches of Cobalt/Palladium Ore. Once you have 45 Cobalt/Palladium Ore, return to your base and use your furnace to smelt them then use your Iron Anvil to make a Cobalt/Palladium pickaxe/drill.

At this point, you should continue exploring the Cavern and Underground layers for Cobalt/Palladium and Mythril/Orichalcum Ore. Your first item to make with Mythril/Orichalcum is a Mythril/Orichalcum Anvil. From there, you want a Mythril Drill or Mythril Pickaxe (Orichalcum Drill/Pickaxe) and a set of Mythril/Orichalcum armor. You need not skip Cobalt/Palladium armor; it is simply a matter of which you get the necessary materials for first. A good starting way to find the ores in to look on your map at different caves you've recently explored. The ores will most likely show up.

If you are playing on the console or mobile version, you may want to save your old armor for the endgame armor made from Souls of Blight.

Hardmode armor bonuses
Hardmode armors work differently from the end-game pre-Hardmode armors. There, you chose between a full set of Necro, Molten, or Jungle armor, depending on whether you were a ranged, melee, or magic character respectively.

Hardmode armors give you a different set bonus based on which headgear you equip. Each of the three basic Hardmode armors has three different head pieces, which correspond to ranged, melee, and magic bonuses.

Which you use is up to you and your weapon selection of choice. The melee helms offers a lot more defense than the others, as well as buffs for melee attacks, so it's a good idea to keep one of those around.

The alternative hardmode armors—Palladium, Cobalt, Orichalcum, Mythril, Adamantite and Titanium—have different helmets that determine armor and give small bonuses to ranged, magic, or melee, similar to the original hardmode armors, and an additional powerful bonus regardless of the helmet you choose: Palladium armor grants health regen when you hit an enemy, Orichalcum sends a 'magic petal' to strike any enemy you've hit, and Titanium armor grants you 2 seconds of invulnerability after you are hit (20 sec cooldown).

The next step would be to make Hallowed Armor. You craft it with Hallowed Bars dropped by the 3 Mechanical Bosses. (The Destroyer, Skeletron Prime, and the Twins.)

Easy Gear
Once you have a set of Hardmode armor, and potentially a Hardmode weapon, it's time to get some very useful and easily acquired gear.

You can enter the Cavern layer now, though you should be on your guard. Skeleton Archers and Armored Skeletons are quite dangerous and also common. Using knockback-inducing gear is helpful for getting the Armored Skeletons away from you. WARNING: Don't be arrogant! These new Cavern monsters (Like the Armored Skeleton) are basically equal and may even surpass you in all stats. Charging will almost always kill you. Never try to tank these monsters, and be tactical, as that is your sole advantage.

Your first goal is to harvest Crystal Shards from the Underground Hallow. They grow naturally on Pearlstone. Indeed, since they give off light, they can be very useful in finding new caves. You need a pickaxe to mine them.

The greatest danger to you in the Underground Hallow will be Chaos Elementals, strong teleporting enemies with lots of health. Also, don't get hit by the Enchanted Sword enemy, as they can curse you, which can be very dangerous unless you have a Nazar, or any accessory containing the nazar.

The purpose of mining Crystal Shards is to make a Phasesaber, which (particularly with good Reforge buffs), is a very strong melee weapon. You need 50 Crystal shards to craft one. You will also need Crystal Shards for Greater Healing Potions, needed for Hardmode boss fights.

Another strong, easily obtained Hardmode weapon is the Dao of Pow. Getting this one requires some preparation. You must build a Desert in the Hallow and Corruption, and then let the infection take its course. Once that happens, Light or Dark Mummies will spawn. These drop the main components of the flail. The other components are Souls harvested from the Underground Hallow and Underground Corruption/Crimson.

Once the Dao is obtained, you will now find that fighting Wyverns is much easier, as flails do lots of damage to them. These drop Souls of Flight, a key component in Wings.

Finally, a very easy to obtain weapon, superior to the Phasesaber, is the Cutlass. To acquire it, one must first get a Pirate Map from enemies close to the sea. The sea is probably the safest place in Hardmode, so this requires no preparation nor skills. Once you have one, go to the middle of the map and build yourself a shelter and a seven tiles long lava pit (one bucket of lava per seven tiles on each side). Summon the pirates and they should kill themselves walking on the lava, letting you collect their loot, hopefully a Cutlass or two with very little trouble.

The quest for Adamantite/Titanium
Once you have a full set of Mythril/Orichalcum armor (and weapons, suited to taste. The Phasesaber is stronger and faster than the Mythril/Orichalcum Sword, though it has less knockback), it is time to search for Adamantite/Titanium Ore.

These ores will only be found in the lower portions of the Cavern Layer. Spelunker Potions can be very handy for spotting these ores. Since these portions also frequently have pits of Lava, Obsidian Skin Potions may also be of some value in mining them. Note that Adamantite/Titanium is only rarely found in the Underworld, and even then only in the upper part of that area.

Crafting every piece of Adamantite/Titanium gear requires 900 ore (980 ore for titanium), since it takes 5 ore to make 1 Adamantite/Titanium Bar. Included in this cost is the 30 ore it takes to make an Adamantite/Titanium Forge, which is also necessary to create the associated bars.

At this point, Cavern layer enemies should be fairly difficult, but survivable. Similarly, you should be able to search through Underground Corruption/Crimson/Hallow for Adamantite/Titanium. This will also give you the chance to farm Cursed Flames (or Ichor if you have Crimson instead of Corruption) and Crystal Shards, as well as Soul of Night/Light. All of these are important materials for the next phase of Hardmode.

Bosses
Once you have all of the Adamantite gear you want, you are now ready to consider taking on the Hardmode bosses. While difficult, it is possible to take these bosses on with Orichalum or even Mythril armor if you can't be bothered to mine all that rare ore.

Before bosses
There are a number of items you should have before you consider taking on any Hardmode boss for the first time:


 * Philosopher's Stone: This lowers the Potion Sickness Debuff timer, allowing you to drink potions faster. For many bosses, this is a vital survival tool. Common drop from Mimics.
 * Cross Necklace: This increases the hit-stun time, thus preventing you from taking damage again for longer. It is vital unless you are good at dodging AND not using melee. Rare drop from Mimics.
 * A full set of Adamantite Armor, with both the Helmet and Mask. Choice of Headgear is optional. This will help you in insuring that Skeletron Prime, The Twins, or the Destroyer won't crush you like a gnat when the fight starts.
 * The Ranger Emblem, Sorcerer Emblem or the Warrior Emblem, dropped by the Wall of Flesh.
 * A Fire Gauntlet (as of 1.3.0.1, unobtainable before killing all three mechanical bosses) to give melee weapons a little extra power as well as making them give off light and hand out fire effects. Most effective against the Destroyer due to its nature for him getting in positions where a sword, spear or mace can slide across many sections per swing.

Two of these items are drops from Mimics, three if you count the Titan Glove used to make the Fire Gauntlet. These are fairly rare, but they can be found in just about any Hardmode environment. You may need to farm these creatures, possibly employing Battle Potions, Water Candles, and Blood Moons to increase their spawn rate. Mimics also drop other useful gear, so time farming them will not be entirely wasted.

The Emblems drop from the Wall of Flesh, which may also need to be farmed. Once you have full Adamantite gear, with the Dao of Pow and the Phasesaber, you'll find that the Wall of Flesh can barely do damage to you.

Hardmode bosses can only be summoned at night. Make sure that it is not during a Blood Moon or other event; you absolutely do not want random monsters adding to your trouble during the fight. Also, Hardmode bosses will leave once the night ends (except Skeletron Prime, which will become nearly invincible and follow you everywhere to kill you), so you want to start the fight as soon as night falls.

The Destroyer
The Destroyer is the Hardmode version of the Eater of Worlds, though it doesn't split into multiple worms. Despite having the highest HP of any Hardmode boss, it is generally considered the easiest Hardmode boss to fight.

Assuming a melee build, fighting The Destroyer can be done by building a box approximately 15-20 blocks wide and 15 blocks high. This makes it less likely for its probes to become involved in the fight.

You should use the Cross Necklace and Cobalt/Obsidian Shield to make damage less relevant. The Phasesaber, especially with any attack speed buffing augments or accessories, is an excellent weapon for taking it on. Movement-enhancing accessories should be removed in favor of defense or damage-based ones. Ironskin Potions are also of value in the fight.

The general strategy is simple. Let it burst up from the ground, then get inside its arc and attack it. Kill any Probes that emerge; their hearts will help you survive without using potions.

The Destroyer's loot now drops from the segment of his body closest to you. It drops Souls of Might, which create key items for the next two bosses.

The Twins
The Twins are the Hardmode version of the Eye of Cthulhu. As the name suggests, there are two of them, and they are a more difficult boss.

It is very difficult to pull off a melee-based strategy with these two bosses, as they fly around a lot. Ranged and magical builds are probably best for dealing with The Twins. Souls of Might are used to craft the Megashark, the strongest ranged weapon available before defeating Plantera. With a good Reforge on it, it is a devastating weapon. If you feel like doing the Twins before the Destroyer or don't want to spend money on buying ingredients, farm the Jungle for an Uzi. However, reforging it is very expensive, so unless you have a lot of money lying around, only do so if it has a negative modification on it.

A good strategy for The Twins is to find a relatively flat area of terrain, use Gravitation Potions to fly, and fight them in the air. Falling up and down makes it much easier to hit The Twins, particularly Spazmatism (the green one). Note that flying too high can cause Wyverns to join the fight, which is something you absolutely do not want to happen.

If you use the Megashark method, you will need a good 1000 Hardmode-tier bullets. Either Cursed Bullets or Crystal ones will do. Cursed bullets do more direct damage, but you can often lure the eyes into hitting the fragments from Crystal bullets. Another, better strategy as 1.2.3 is to use a combination of Explosive Bullets and Ichor Bullets (If in a Crimson world) or Cursed if in a Corruption. These make the fight much easier, unless you let them stay too close, killing even Hallow Armored players in moments. Also, Crystal Shards are rather easier to obtain than Cursed Flame, but still more difficult than Explosive Powder, assuming a reasonable supply of money. You should obviously use the Adamantite Mask for its ranged buffs and increased chance to not consume ammo.

The Cross Necklace, Philosopher's Stone, and Cobalt/Obsidian Shield will again serve well in this fight. The Ranger Emblem will also maximize the utility of each bullet. When using the gravity potion method to fly, wings will be helpful in case you forget when the potion wears off. If you are certain of your ability to keep the Gravitation Buff active, then you can swap out the wings for another defensive accessory.

Skeletron Prime
Skeletron Prime, as the name suggests, is the Hardmode version of Skeletron. He comes in four parts plus his head, each with their own health.

Note that, like Skeletron, Skeletron Prime will not simply leave when morning comes. It will instead become nearly invincible (gaining 9000 defense) and start spinning, with a one-hit-kill attack. So make sure to start the fight as soon as night comes to give yourself enough time.

The general idea with this fight is similar to The Twins fight: fly around and use ranged attacks. However, with access to Souls of Sight, the Hallowed Repeater becomes available. This offers a choice between bullet-based attacks and arrow-based ones.

The Hallowed Repeater benefits from the Archery Potion buff, but it attacks slower than the Megashark, so you will be more likely to hit him with the Megashark. However, Skeletron Prime and its parts have lots of defense, so doing more damage per shot can kill him faster if you have good aim. Offsetting this, the Megashark uses less ammo, since it has a 50% chance not to consume ammo per shot. Either one is doable.

When using the Hallowed Repeater, Cursed Arrows or Ichor Arrows should be used.

The usual set of boss-fighting accessories is valuable: the Cross Necklace, Philosopher's Stone, and Cobalt/Obsidian Shield, with the Ranger Emblem, though if using the repeater, the Magic Quiver may be a better option.

Flying with a Gravitation Potion or fighting over Wood Platforms removes the Prime Cannon as a threat, since its grenades only explode on contact with solid blocks (Or players so avoid contact). The Prime Saw is the biggest initial threat, as it does lots of melee damage. Killing it early is a good idea.

With the Souls of Fright dropped from Skeletron Prime, you can craft a full set of Hallowed Armor. You may also craft the Drax, Flamethrower, and other useful items.