I can do one worse, ran Onslaught this past week, normal mode, killed the wave 10 boss and no chest spawned and the portal never opened. We waited, we wiped a couple of times (which BTW took us back to the entrance portal spawn sight), waited, tried what little we could before finally just having to leave. The only brightside it was only wave 10 and not 40 or worse yet 50! Well another brightside, found a semi-decent hiddy hole that did not require jumping up and out of the arena. That's how long we waited, got lots of exploring time in!