Stargazer

There is no summon item. It spawns around the stardust pillar during the lunar invasion, which only begins after the cultists at the dungeon are dead, so every one you meet is part of clearing that pillar's shield.

It walks slowly and does nothing until it has a clear line to you, then it plants itself and fires from its chest. The beam arrives first as a thin harmless line, and that line is your warning: it is drawing exactly where the damaging version is about to be. You get a moment to leave, less than a second, before it widens and stays there for about another second. Move across the line, never along it. The first shot after you enter its range comes up faster than the ones after it, which is what catches people walking in.

Every hit you land pushes its next shot back, so a fast weapon can keep one silent almost permanently. What a hit will not do is cancel a beam already out. It also cannot fire while it is off the ground, so anything that knocks it into the air buys the same time as damage does. Strike it mid beam and it starts walking again, dragging the beam with it, but the angle stays locked, so the beam sweeps across the ground rather than tracking you. That sweep is easy to dodge once you expect it, and it can carve up your own arena floor plan if you fight in a tight space.

The beam runs an enormous distance through open air, well past what fits on your screen, which is why these clip you out of nowhere while you are busy with the rest of the pillar. Solid blocks stop it dead. A few short columns scattered through your arena break the sight lines and turn this into the least of the problems at that pillar. It is also stationary for a full two seconds around each shot, so it is the easiest kill there if you can spare the attention.