Skeleton Archer

They start appearing in the Cavern layer once the world flips to Hardmode, on their own, with no summoning involved. They walk slowly, so the approach is never the danger. The danger is that they open fire from most of a screen away, long before you have decided whether you want the fight. Unlike Armored Skeletons they cannot work a door, so a door genuinely holds them out.

It stops walking the moment it has a clear line to you, then takes a beat to line the shot up. Land a hit inside that beat and the arrow never leaves. After that, each hit you land pushes its next shot further back than the shot cycle itself, so a fast weapon can hold one in a permanent aiming pose until it dies without ever firing.

They burn, so damage keeps ticking after the hit lands, and standing in water will not put the fire out the way it does for most burning effects. You cannot pick any of them up either, so there is nothing to recover from being shot at. Across an open cavern, where early Hardmode gear often cannot reach back, that adds up quickly.

A pit around seven blocks deep on your usual route handles these and Armored Skeletons in one go. Neither can climb out, and neither can shoot what it cannot see, so they pile up harmlessly until you feel like collecting the drops.

It carries a plain wooden bow and hands you a Marrow instead, and there is no quiver anywhere on the sprite despite the Magic Quiver being one of its drops. The armor is a recolored ancient cobalt breastplate with spikes added at the shoulders.