Etherian Lightning Bug

It exists only inside the Old One's Army, and only in the top tier of it, so Golem has to be dead before you will ever see one. It comes in on the second wave and again on the sixth, which bookends the run: once while you are still settling into the rhythm, once when the crystal is already taking pressure from everything else on the field.

It flies to just past the end of your swing and shoots from there, roughly twice a second, and every bolt halves the damage you deal for the next few seconds. Because the shots barely stop, that halving is effectively permanent for as long as one is alive. A melee player who keeps swinging at empty air loses the wave without the crystal ever being touched. Close the gap yourself or bring something with reach, and kill it before anything else in the wave.