Guide:Skeletron Prime strategies

Skeletron Prime, the Hardmode version of Skeletron, is intended to be the most difficult of the three Mechanical Bosses, though can be considered easier than the twins. His high health, defense, damage, and multiple limbs will take a lot of punishment, and his limbs have the added effect of blocking non-piercing shots aimed at his head.

Terrain preparation
As with all bosses, an arena made out of Wooden Platforms greatly increases your mobility. You will want 10-15 rows of Platforms, each around 50 or so blocks long. The platforms should be suspended off the ground to prevent the Prime Cannon's grenades from ever detonating. A solid ceiling should be avoided since Skeletron Prime will often retreat a fair distance when not using his spinning attack. Every 5 or so rows, replace the centremost Platform with a solid block of any kind and place a Campfire and a Heart Lantern on it. These two items will greatly boost your health regeneration up to 10 hearts a minute, even while moving. As with most bosses, starting the battle as soon as the sun sets will provide the maximum amount of time to beat Skeletron Prime which is vital since defeating him can take all night. If you do not, he will transform into Dungeon Guardian Prime, making him instantly kill you and take nearly no damage. A honey buff will help a good amount. And if you place heart statues with wires on them and hook them up to a 1-second timer it will give you an endless supply of hearts.

A circular arena of mine tracks with a campfire and heart lantern in the middle also works (including expert mode).

General
Adamantite armor or Titanium armor with your respective helmet is a good set. Accessories that increase your mobility such as the Spectre Boots or Wings or the Jetpack bought from the Steampunker are pretty much a must. If you have farmed The Destroyer or The Twins, Hallowed armor becomes a great choice. If you can, farm the Wall of Flesh to get your respective emblem, be it ranged, melee, magic or summoning (provided you're on PC). Also Getting Life Fruit is a good idea as it can be farmed and increases your health to a maximum of 500. For melee users, the Dao of Pow is an excellent weapon to use due to its long range and damage. The Ice Sickle (the Death Sickle can be used on console if one mechanical boss has been defeated) fires a spinning scythe projectile that will pierce Skeletron Prime and can be acquired from mobs in the Underground Snow biome. Some type of spear weapon, ideally the Chlorophyte Partisan, Adamantite Glaive or Titanium Trident, can hit it if you get close enough. You can also use the Bladetongue which fires an Ichor projectile provided you can get close enough to it. The Flying Knife is quite hard to acquire (being a rare drop from Hallowed Mimics) but it does not require to be in close quarters for full damage. Once you've acquired Hallowed Bars, the Gungnir and Light Discs with their high range can be helpful. Also, if you manage to acquire them, Bananarangs are a very effective weapon, due to their high damage and speed. Frost armor will boost your damage and can inflict the Frostburn debuff. The shadowflame knife is also an excellent choice. If you have good armor, use a fast sword like the Cutlass with high damage and good modifiers. This can take down the head and limbs easily, provided you can absorb and/or dodge attacks.

For ranged users, you can get your hands on the Adamantite or Titanium Repeater which are easily obtained. Combine them with Holy Arrows or Frostburn Arrows (Frostburn Arrows will inflict the Frostburn debuff) for acceptable damage rates. However, there are better alternatives; the Clockwork Assault Rifle is a burst-fire weapon with high speed which is complemented by Crystal Bullets. The Daedalus Stormbow is extremely powerful - especially when combined with Ichor Arrow, Hellfire Arrows or Holy Arrows - but quite hard to acquire, being a rare drop from Hallowed Mimics. However it is very easy to farm the Daedalus Stormbow and is worth the time. If you have defeated The Destroyer, you should definitely consider the Megashark due to its insane speed and respectable damage. Frost armor will boost your damage and can inflict the Frostburn debuff.

Magic users can use the Crystal Serpent which functions similarly to Crystal Bullets and deals high damage. The Crystal Vile Shard deals somewhat low damage but can be used to hold Skeletron Prime's limbs in place, which allows the damage to stack up if you keep firing. If you have defeated the Twins, the Rainbow Rod is effective as it does 70 damage, quite a lot if you haven't defeated the 3 mechanical bosses.The Meteor Staff is extremely powerful and easy to craft, provided you have some leftover Meteor bars from Pre-Hardmode (Note the Crystal Serpent, Meteor Staff and the Crystal Vile Shard are all 1.3 only.) The Golden Shower deals little damage but inflicts the Ichor debuff even when it is switched away, allowing your other weapons to deal even more damage. 20-30 Greater Mana Potions are suggested depending on your mana usage (To combat the Mana Sickness debuff, have some Star Statues placed a regular intervals linked up to Three Second Timers. Whether they are on the ground or on a platform shouldn't matter unless the terrain is full of hills. Flatten the area around the statues to make mana regeneration easier. Alternatives can be using the Hallow armor set's magic variant paired with the Crystal Storm with a good pair of wings with good mana accessories with either Menacing, Warding or Arcane can enable you to defeat Skeletron Prime easily.

Summoners can use the Optic Staff to summon miniature versions of The Twins. There's also Pirates from the Pirate Staff, but those lack the ability to fly. Neither Adamantite armor nor Titanium armor have helmets that boost summon damage but you can brave the Spider Nests for Spider armor which is designed exclusively for summoners. Alternatively, farm the Desert biome until you have enough Forbidden Fragments to make Forbidden armor which allows you to use the Sand tornado ability, which can do extra damage to Skeletron Prime, allowing you to finish him off quicker. The Queen Spider Staff is a great choice if you tend to remain in one vicinity the entire battle.

Potions
Bring along 10-15 Greater Healing Potions (don't even consider regular Healing Potions, you should have made some of the Greater variety already). Mobile users can also use Horn o' plenty if you have beaten Turkor the Ungrateful. For buff potions, the classic trio of the Swiftness Potion, the Ironskin Potion and the Regeneration Potion will do you a world of good. Endurance Potions will complement the defense boost of the Ironskin Potion. Rangers with Bows should bring along an Archery Potion or two. Mages should use Mana Regeneration Potions and Magic Power Potions. Damage increasing potions like the Rage Potion and the Wrath Potion are good for everybody.

The Battle
Skeletron Prime is a flying boss that consists of five parts: If you take out his head, you kill the boss.
 * Prime Saw, a melee arm, with 9000 health.
 * Prime Vice, a melee arm, with 9000 health.
 * Prime Laser, a ranged arm that fires lasers, with 6000 health.
 * Prime Cannon, a ranged arm that fires grenades, with 7000 health.
 * Skeletron Prime, the head, with 28000 health.

Each limb has its own unique attack pattern, listed below. Although killing the limbs is not necessary, they can block the head (especially Prime Saw and Prime Vice), and destroying them can make the fight much easier. It's suggested that you should spend half the night attacking its limbs, then spend the other half of the night attacking the head.

Prime Saw
Prime Saw is Skeletron Prime's most prominent melee attack limb, and the one that will do the most damage of any limb. At regular intervals, it will chase down the player, causing serious collision damage each time it hits. The arm can move as fast as the player can with Spectre Boots, and will accelerate in short bursts so it can catch up. It will eventually pull back to the head, waiting to repeat the cycle.

As it does a lot of damage, this arm should be destroyed first. By using the wooden platforms to change height when it charges, or with great speed, Prime Saw's attacks can be avoided until it is destroyed. Using a melee armor set will help mitigate damage taken, after which a ranged set can be equipped.

Prime Vice
Prime Vice is Skeletron Prime's secondary melee arm. Though it will occasionally chase the player, it is more common for the arm to simply charge directly at the player, retract, then attack again. This arm is relatively easy to dodge, making it a minimal threat to an agile player.

Prime Laser
Prime Laser will consistently remain above the player, firing one laser every second or so. The lasers aren't as damaging as melee attacks and can be avoided fairly easily, but are harder to dodge if attempting to damage the head. Its low defense and health make destroying it much easier than the other limbs, so it may be worth removing simply to make the head easier to target.

Prime Cannon
Prime Cannon fires grenades at a rate faster than Prime Laser, which also do more damage. However, these grenades only detonate on contact with a solid block. In a wood platform arena, this effectively renders Prime Cannon a nuisance that occasionally blocks attacks. Leaving it intact buys more time to defeat the head.

Skeletron Prime
Skeletron Prime is the head, which must be destroyed to win. Skeletron Prime will try to float above the player at all times (use this opportunity by shooting the head from below), high enough that he may be entirely off-screen at lower resolutions. He'll close in to do his spinning attack, which doubles his attack and defense. Once he finishes spinning, he'll retreat again. If the head is not destroyed by morning, it will be locked in its spinning attack, gain a speed increase, and its attack and defense will increase to near-unbeatable levels. In this state, a single hit will kill the player, while the head will only take one damage per hit, and two damage per critical hit.

When trying to destroy the head, it helps to have destroyed at least two of its limbs first, ideally one melee-based and one ranged-based. The remaining two usually won't block too many of your shots. If all of the limbs but his head have been destroyed, he becomes incredibly easy. His attacks are relatively simple to dodge, and though he can cause a lot of damage, getting hit once or twice isn't a very big deal since he doesn't attack too frequently. The wings and Spectre Boots make it almost impossible for it to hit you.

When Skeletron Prime stops attacking and tries to retreat, chase it down using wings. Though they'll eventually give out, you'll do a fair amount of damage on the way up. The head will come back down fairly quickly. Attack the head constantly and it should be destroyed before morning.

Caution: Even with the best armor and defense possible, Skeletron Prime can still easily kill you, as the head's damage while spinning can only be reduced to 53.