I see a few topics sort of like this, but not one exactly like it, so here goes. I have the same problem with Cistern of Slaughter and the Slaughter Shaft–I get deep into the rounds, doing fine, and bam, one or more mobs gets blasted into the geometry or out of the arena. The wave won’t end, of course, so I have to repeat it. Of course, it happens usually on the last wave of the last round before the final boss. I am kind of tired of redoing these, especially Cistern, where the legendaries don’t drop as much as with the Shaft, where they drop like rain.
Naturally, I’m using Amara with either a richocet or an ion cannon build, so there’s a lot of blasting.