Rock Golem

Cavern layer, hardmode, and only ever on stone. Cut a tunnel through a mud or dirt pocket and none will spawn there however deep you are. Snow country is the flat exception: the game will not place one in the Ice biome even where bare stone is exposed. Nothing summons it, so encounters come from simply spending time in ordinary stone caves after the world turns.

It throws rocks at anything it can see but cannot reach, so the fight splits by distance. Stay back and you spend it dodging stone down a tunnel. Close the gap and the throwing stops, but now you are beside something heavy that will not flinch. Hitting it interrupts nothing, so the usual trick of walking up and holding attack to keep a cavern enemy quiet does not work here. Kill it quickly or keep moving the whole time. Tight corridors are the worst ground to pick.

For the Worthy and Zenith worlds add a parting shot: a Boulder drops loose when it dies and rolls off down the tunnel, so finishing one while standing below it can undo the fight you just won. It is also one of only two things in a world that produce fresh stone, gem trees being the other.