Mysterious Portal

Two of these open the moment an Old One's Army wave begins, one on each side of the Eternia Crystal and set well back from it. Every enemy in the event walks out of them. You cannot break them, they ignore damage completely, and they only close when the event ends, whether you held the crystal or lost it.

Since you know exactly where things will appear before they appear, put your damage there instead of around the crystal. A trap laid at a portal mouth, or a lingering effect like the Clinger Staff or Rainbow Gun parked across the exit, hits everything the instant it steps out. A defense stacked on the crystal fights the same enemies later, more spread out, with more chances for one to slip past you.

The Demolitionist has a line about how much simpler all of this would be if someone would just let him blow them up.