Paladin

A Hard Mode miniboss that spawns in the Dungeon.

He throws hammers at you from range. Hammers pass through walls. Constant damage appears to stop him from throwing hammers. Items such as Vilethorn or the Crimson Rod seem to be best for this.

Caster based strategy
 Items required:
 * Crimson Rod or similar
 * Low mana cost bolt spell (Diamond Staff for example)
 * High end caster gear (Hallow or better, preferably)
 * Any grappling hook

Execution:

  When you see the Paladin coming, you need to prepare fast. Make sure you have all the required gear easily selectable. Grapple anywhere where you can sit above him easily and once he's below you, cast your Crimson Rod right above him. The constant damage will stop him from throwing his hammers up at you. (Presumably. That's what it seems like.) Make sure to maintain the raincloud to keep the constant damage coming. This is important or he WILL throw hammers at you and kill you quickly.

Once you've established automatic constant damage, spam your bolt spell down at him to deal buckets of damage. Be sure to pause around half mana to regenerate so you can keep spamming your bolt spell! Be sure to bring health potions as well for when  the damage interval is long enough for him to throw a hammer. Also, be sure to stay on your toes for any of the Dungeon's normal mobs. Those mage skeletons hurt!

Good luck!