How do I defeat the Dungeon Guardian?

Lawtonfogle's answer works if you have two people, but if you want to do it solo, there are two ways: Note: Lawtonfogle's answer no longer works as of patch 1.2.0.3.

• Actually fight him. (This is now the only way as of 1.2.0.3.) You should use Gravitation Potions, a Megashark (preferrably Unreal), and Crystal Bullets (so that you can hit multiple times because of the shards that fly out of them). Also have Spectre Boots, so that you move as fast as possible. (Spectre Boots, unlike Hermes boots, still give you the speed boost while flying.) The new Lightning Boots would be preferable; they go even faster.
• Use Turtle Armor and Black Belt, and there is a chance the Black Belt will let you dodge the first hit, while still bouncing back damage. Then on the second hit, you will die, but the Guardian will die too. (Of course, there's a very small chance that you'll survive two hits, leaving you unharmed and one Guardian dead.) Note: no longer works in 1.2.0.3. The Guardians now only do 1000 damage, so it takes 10 bounce-backs to kill them.

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You use turtle armor with 2 friends. Turtle armor does full damage back to the attacker. Two of you being killed while wearing turtle armor should kill it with the last guy picking up the drop. Good luck getting the armor.

Here is a video showing how it can be done. They use a black belt and a lot of luck (and a lot of tries) to dodge the guardian twice in a row while it kills itself due to the turtle armor.