Skeleton Commando

It only exists in the Dungeon, and only after Plantera is dead. Nothing summons it. It is one of the rarer things down there, and it strongly favors the rooms backed by brick walls, so sweeping those sections repeatedly beats wandering the whole place. A Lifeform Analyzer will call it out before you see it, which is worth an accessory slot given what it drops.

Its firing range is wider than your screen, so the first warning is usually a rocket already in the air. The explosion covers a decent square around the impact point, which means being near where it lands is enough to take the hit. In a narrow Dungeon corridor that blast has nowhere to go except into you. It walks slowly, so if you are getting hit, the fix is almost always to break the line of sight rather than to run.

It has to plant itself with a clear view of you before it can fire, and every hit you land pushes the next rocket back further than its normal cycle. Anything with a decent rate of fire keeps one silent from the moment you find it. It also cannot shoot while it is in the air, so catching one dropping between platforms buys a free moment. The first rocket has a short windup as well, and interrupting that means it never gets a shot off at all.

The Rocket Launcher is the reason. It carries a ranged character through the stretch right after Plantera, and on Expert there is a small chance of walking away with two at once. They stay in the Dungeon rotation permanently once Plantera is down, so there is no window to miss.