Barnacle Staff

Summoning it does not place it where you clicked. The barnacle rises under its balloons until it either hits a ceiling and clamps to the underside, or gets far enough above the ground below it to stop climbing. In open sky that means it drifts high and spits down from a distance. Under a roof it pins itself where the roof is, which is usually where you wanted it anyway.

Once you know the behavior it is easy to control. Put a flat ceiling over your arena and the sentry parks in exactly the same place every time you summon it. Break or actuate the block it is holding onto and the balloons come back and it goes looking for a new one, so avoid actuated floors in the room you plan to fight in.

It fires on its own without any attention from you and keeps the poison ticking while you focus on dodging, and its targeting range covers a whole arena from one placement. The catch is entirely in how you get it. It comes off a fishing line in the Hardmode Jungle at odds nobody would farm on purpose, so treat it as a free addition to a setup you already have rather than something to build around.