Gastropod

Nighttime, the hallowed surface, hardmode. Nothing summons them. They simply spawn thickly once the sun is down, and several will usually have line of sight on you at the same time. They fly, and unlike most flying enemies they will follow you into water and keep shooting from under it, so a moat buys you nothing.

There is a short animation before every laser: the shell splits open, then the shot leaves. Hit it inside that window and the shot never happens. A weapon with a high rate of fire keeps one stuck in the opening frame more or less permanently, which is the whole difference between a harmless snail and a beam that crosses the screen faster than you can react to it. The Bee Gun is the classic answer for exactly this reason.

They keep traveling for as long as they have you in view, and it is the speed rather than any single hit that gets people. Recent versions gave them some built-in inaccuracy and stopped them firing at players who are far off, which took the edge off the worst of it, but at normal fighting range they still land more often than they miss.

Stealth does not save you either. Shroomite and Vortex stealth stops nearly everything from tracking you. A gastropod will still aim at you while you are invisible, even though it will not move toward you. Assuming you are hidden is an easy way to die in a state you thought was safe.

A fresh hardmode character has no business on the hallowed surface after dark. Not because any one of them is dangerous, but because four firing at once is more incoming damage than that gear can absorb. Royal Gel fixes it outright, since they count as slimes and stop caring that you exist, though that accessory only turns up in expert and master worlds.

No other non-boss slime hands over Gel at anything close to this rate, which matters the moment you start feeding a Flamethrower or an Elf Melter. Two things they do not do, despite being slimes: they never leave a Slime Staff, and killing them does not tick anything off the slime variety achievement.