Title says everything. What armor should I have? What weapons?
Destroyer is very dependent on your weapon choice, more than almost anything.
As for armor, you should have Adamantite or Titanium as melee or mage, Frost as ranged, and Obsidian as summoner. Forbidden might be good if you want to hybrid mage and summoner, and its tornado is good versus Destroyer.
You really want weapons that either have pierce or projectile flood. Weapons like these will crush the Destroyer in no time, while other weapons will struggle.
For Melee, your best choices are flails: Dao of Pow or Drippler Crippler. You can also use Fetid Baghnakhs to tank the boss, but this isn’t a good strat in higher difficulties.
For Ranged, your immediate best option is Dart Guns with cursed darts. Daedalus Stormbow with wither Jester, Exploding, Unholy or Holy Arrows is also good, and can deal with the ensuing Probe spam.
For Mage, Life Drain and Golden Shower are your best bet, Meteor Staff if you’re corrupt and don’t want to use opposite evil weapons. Clinger Staff and Nimbus Rod should be used as support for all classes. You can also melt him with Tome of Infinite Wisdom if you’ve already done OOA.
For Summoner, the OOA sentries (Explosive Trap) can decimate him, and any whip does decently against him. Minions won’t make up much of your damage on this boss specifically because they don’t have linear pierce and the whip tags won’t stay, so I recommend something that attacks the nearest enemy automatically to deal with the probe swarms - Sanguine is good here.
Your arena should be a platform raised above the ground so you can see Destroyer coming and have a greater amount of his body open for attack at once. If you use any of the weapons that rain from the sky, you can put a box about 60 blocks up and be basically untouchable, but this is more of a cheese. You’ll want Heartreach Potions to pick up the probe drops along with traditional combat potions, if you need them. But overall, Destroyer is easy enough with the weapons I listed to where this shouldn’t be an issue.