I’ve been slamming my head on this dungeon for a couple of hours now, and have gotten really good at killing 4 sets of 15+ mobs, but when it comes time to kill the remaining mobs, the Dungeon either stops or cuts out my remaining time to kill mobs. Or upon killing the final mobs it doesn’t register the final Mastery achievement. Timer expires specifying: Target time 5:00/Your time 3:40.
Maybe you have taken fire damage during the dungeon. Just guessing. 