Angry Bones

Ordinary walker AI with two habits that make the dungeon awkward. They break through doors, though it takes a hit or two, so sealing a corridor only buys time. And they leap, usually when you get close or stand above them, which catches out anyone who assumes a one tile ledge is safe. The dungeon feeds them in from side rooms constantly, so you are rarely dealing with one.

Everything the dungeon hands you happens to be good here. The Water Bolt ricochets down a corridor and rakes the whole line, and the Blue Moon sweeps through several bodies per swing, which matters more than raw hitting power when four of them are queued in a hallway. If they get on top of you, a hook into the ceiling puts you above the jump arc and lets you fire down into the group.

Two of the variants carry enough health to qualify as a Dungeon Spirit source once Plantera is down. That turns the plain old part of the dungeon into an Ectoplasm farm rather than something you clear once and abandon, so think twice before walling it off.