Guide:The Destroyer strategies

The Destroyer is one of the bosses unlocked when the game enters Hardmode. It is extremely strong, dealing very high damage to the player even when the player has high-tier armor. It is generally recommended as the first boss to fight following the Wall of Flesh.

Terrain Preparation
The type of preparation you'll be making depends on your approach. If you are using melee, The Destroyer is best fought on an arena consisting of tall walls with multiple layers of Wood Platforms. This will allow players to move around the arena freely, and to keep a good watch on where the boss is. If you are using ranged, pick a wide, open, flat area, with plenty of clear space for flying and dodging. The Destroyer can clear a lot of vertical space. Like any other burrowing enemy, it can only control its movements in tiles/blocks.

It may be useful to summon and kill Eye of Cthulhu (he will die in a matter of seconds when engaged with hardmode weapons) before summoning the destroyer in order to spawn an extra one-use pool of Hearts. You can also choose to add a few Heart Statues wired to a Timer to supply you with some additional health. Another great help is building a few small pools of Honey and building Campfires and Heart Lanterns around the arena. Last of all you can move the Nurse in a nearby house (not too close!) for some in combat healing.

A tricky alternative way of building an arena is by creating a small box, wire up a few Heart Statues around it and some honey inside it. Combine this with high defense, a Cross Necklace and a Charm of Myths. The Destroyer himself will only deal relatively low damage, less than your healing potential, making the fight very easy. Another way is to build a vertical platform very high in the sky (200 to 300 blocks), with 1 block replaced by a Wood Platform. The Destroyer will be unable to reach you, allowing you to easily kill him with ranged weapons. Although those two ways may make the fight a lot easier, they might also make the fight somewhat boring.

Armor
Mythril armor or higher is highly recommended for this fight, as The Destroyer hits quite hard. Note that full Adamantite armor will significantly reduce the damage from body segments, making this boss fairly easy to solo if you do plenty of mining first. But remember that it's always good to fight bosses [The Destroyer,ect.] with friends/other players. Titanium armor is also incredibly effective against the destroyer, because of the invincibility after hitting an enemy. However, the player will need to take a lot more time gathering resources to make a full set of titanium armor. Another great option would be Frost armor. It has a high defense, is less time consuming to gather (assuming you can get a Blizzard event). Note that as of 1.2.3, Frost Armor must be crafted using a Frost Core and Hallowed Bars, requiring you to have already defeated a different hardmode boss. A last great option is Orichalcum armor, because of its set bonus. Flower petals frequently multi-hit The Destroyer upon flying across the screen, and can sometimes (if angled correctly) hit most of it's segments length wise.

Weapons
Different weapons can have different usefulness against this boss. However due to his high health and multiple segments piercing weapons are required. Aside from choosing the right weapon it may be worth your time to earn some money and get a good modifier for your weapon as well.

Melee
The Destroyer is the easiest of the 3 mechanical bosses for a melee character, considering his movement in the air is limited.
 * Spears have great potential, especially the Mushroom Spear, but due to 1.2.3, it requires a hardmode boss to be defeated. The Chlorophyte Partisan (You need to kill all three Mechanical Bosses first though) can also be very useful because of its immense range and spore cloud that penetrates walls and can multi-hit a lot of The Destroyer's segments.
 * The Dao of Pow can do significant damage across multiple body parts.
 * An Ice or Death Sickle would also be able to deal significant damage, while allowing you to maintain some distance from The Destroyer.
 * A large sword would be reasonable effective, or used as a backup for defeating the Probes.

Ranged
As with most bosses ranged weapons would be easier to use than melee weapons. And even while this is one of the easier bosses for melee, a ranged weapon with enough piecing can simply hit so many parts at once that it outdamages melee weapons by a great amount.
 * If you happen to obtain a Staff of the Frost Hydra this would be a major advantage, being able to summon the Frost Hydra once and then switching to another weapon, while the Frost Hydra supports you with additional damage and the killing of Probes. Additionally the Nimbus Rod can be used the same way.
 * Various other piecing magic weapons could obliterate this boss in no time. Most notably the Rainbow Gun, the Poison Staff, the Golden Shower, and especially the Heat Ray due to its infinite piercing ability. However if you are unable to obtain any of those you can settle for a Diamond Staff or Frost Staff as well.
 * An Adamantite Repeater or Titanium Repeater, combined with Jester's/Holy Arrows or alternately Unholy/Hellfire Arrows would deal quite reasonable damage.
 * One of the easier to obtain weapons that is still rather effective would be the Clockwork Assault Rifle. This combined with Meteor Shot or Crystal Bullets would work rather well. A Minishark or preferably an Uzi could be used instead.
 * If you already defeated one of the other mechanical bosses you may consider using a Megashark, Flamethrower or Magical Harp.
 * Another interesting weapon is the Cannon, which should be placed instead. It can deal high damage and piece through multiple parts, giving great damage potential, however it may be hard to get in a good shot, aside from aiming up and simply waiting for the destroyer to charge through you.

Accessories
There are many great accessories available at this point. Especially (Frozen/Leaf) Wings would be of great help, as well as Lightning Boots. Those two accessories would allow you to move almost freely through the air and on the ground, making it a lot easier to evade The Destroyer and his lasers. A must have for melee users, as well as being useful for ranged users, would be the Obsidian Shield, preventing you from being knocked around by the lasers, both from The Destroyer and the Probes. The Mana Flower is less useful during this fight, as the Probes you kill will almost always supply you with enough mana and drinking a mana potion once during the fight isn't a great deal either.

For the remaining accessories the Cross Necklace (or Star Veil) are extremely useful due to their ability to lengthen the time delay until you take more damage, especially since you may always be taking damage. The Charm of Myths will also help you survive longer because of its regeneration and potion sickness decreasing effect. If you rather go for more damage an Emblem (or Mechanical Glove for melee users) would supply you with additional damage. As with all boss fights it is best to reforge your accessories with damage or defense increasing modifiers.

Potions

 * As with all boss fights a good supply of Healing Potions or even Greater Healing Potions would help you survive longer and a supply of Mana Potions in case you are going to use magic weapons.
 * One of the most useful potions would be the Ironskin Potion, helping you survive longer. Regeneration Potions also help for this.
 * A Bowl of Soup, Pumpkin Pie or Cooked Marshmallow can never hurt and are quite easy to make.
 * A Hunter Potion could be useful, so you can see where The Destroyer is going to surface and a Swiftness Potion to evade him.
 * In case you are going to use a repeater the Archery Potion is a must have.
 * If you prefer magic weapons instead an Magic Power Potion, Mana Regeneration Potion should be used instead. Also the buff from the Crystal Ball would give you some advantage.
 * If you are going to use melee the Thorns Potion could come in handy as well as a Flask of Ichor which you should now have access to.
 * Shine Potions and Night Owl Potions are helpful, but not really essential. They just make sighting other mobs and the terrain easier.
 * Featherfall Potions and Gravitation Potions are also useful if you want more control while in the air or have limited access to the flight accessories.

The Fight
The Destroyer is almost exactly the same as the fight with the Eater of Worlds, in that it digs around and launches up at you every chance it gets. Just sidestep the body and keep wailing away at it. Occasionally it will launch a few volleys of Death Lasers at everyone fighting him. These hit for about 15-20's on players with 40-50 defense. The Destroyer also spawns Probes throughout the fight. It spawns them very often at first, and then the spawn rate slows down a good bit as you kill them off. Probes drop a 20HP Heart when killed at a 33% drop rate, so for the most part health isn't too much of an issue with this fight.

The Probes are both a blessing and a curse. A curse as they get in your way and can attack you for decent damage with their lasers (a bit more if you collide with them). They are a blessing in disguise, however, as they can be a source of health by killing them when you have less than full health: They can drop Hearts for later healing. Careful management of Health Potion usage and Heart collecting will keep you nice and stocked on health. Just drop to the ground when it's relatively clear and gather some. The Hunter Potion will let you see The Destroyer in the ground, so you'll know if you have to evade instead or can keep going for the hearts.

If you're fighting on a wide, flat plain, it might pay to occasionally just run across it until The Destroyer appears. Doing so might cause it to spread a large portion of its body along the ground, making it an easy target from above. If you have the horizontal space (and speed), make use of it by flying a decent distance back and forth instead of only staying in a particular (horizontal) area to help spread its body out. The more of the body you can keep exposed (and accessible to attacks), the more opportunities you have to deal damage.

On the other hand, if you want to hit the ground (say for gather Hearts), you can fly straight up and hover a while until The Destroyer jumps straight up at you. You can then drop to the ground quick to gather what you need/want while the The Destroyer recovers from its attack attempt. It takes a while for it to loop back around from a high jump. Remember, outside of solid blocks/tiles, burrowing enemies have little control of their movement. This give you the time to take a breather or collect some pick-ups on the ground.

Like most other bosses, if everyone is killed The Destroyer leaves. The Destroyer will also leave if you cannot kill it before sunrise. After a few minutes of bashing away at its health, the machine will fall, but be warned that the loot spawns at the head when killed. Since Souls are not affected by gravity, this can make them difficult to reach after the fight has ended (finding them is easy, as they are above the rest of the loot). It may also happen that his head is deep in the ground, making it nearly impossible to find the loot. It is therefore recommended that once he is nearly dead you wait for his head to surface before killing him. In 1.2.3 the Destroyer's loot drops at the closest section to the player when it dies, so you don't need to worry about this anymore.