Paladin

It appears in the dungeon and nowhere else, and only after Plantera is dead. Spawns concentrate heavily in the brick walled sections rather than the older stonework, so hunt there if you want one on purpose. It is rare enough that a Lifeform Analyzer announces it, and both the banner and the bestiary entry complete at half the usual kill count.

Nothing about it can be pushed around. It ignores knockback completely, carries more health than the boss guarding the dungeon door, and matches some endgame bosses for defense. Its hammers pass straight through terrain, which removes the usual option of ducking behind a block and waiting. Slipping past one is normally worse than fighting it, since it keeps throwing at your back while the rest of the room closes in.

It cannot throw while walking, and it cannot throw while it is being hit. Both are exploitable. A weapon that lands hits continuously locks it out of attacking altogether: the Piranha Gun is close to a hard counter, and piercing magic such as the Nettle Burst, Wasp Gun, Magical Harp or a Shadowbeam Staff does the same job. Minions do it hands free, which is the real argument for bringing the Tempest Staff, Raven Staff, Desert Tiger Staff or Terraprisma into the dungeon. They hold it quiet while you handle the casters around it.

The second opening is sight. It mostly gives up on anything it cannot see, so a closed door, a wall, or even a one tile gap turns it into a target dummy. Anything that reaches through blocks, like a Death Sickle, or fits through a narrow hole, like the Nimbus Rod or Scourge of the Corruptor, kills it for nothing.

Do not take one in a cramped hallway. There is no room to dodge and the hammers are not something you can eat repeatedly. A long two way corridor works if you retreat while firing, but the real hazard is another dungeon enemy drifting into your line and soaking the shots that were keeping it interrupted. It also looks four tiles tall and still squeezes through a three tile gap, so a low tunnel is not an escape route.

In classic mode its body is the safer half of it and the thrown hammer is the thing to respect. Expert and master invert that, and walking into one becomes at least as bad as being hit by a projectile. Expert additionally lets it despawn even after it has picked up coins, so a kill you were carefully setting up can simply walk out of the world.

Money, mostly. Nothing else in the dungeon leaves as much behind, and across the whole game only a couple of non boss enemies beat it. It also visibly carries both of the items it can drop, one in each hand, so you can see the loot before you have earned it.