Your suspect number 1 is correct, I've tested that long time ago. The achievment is not granted if 2nd boss got killed while one of the players was trapped in bubble. This is clearly a bug, but it also has clear workaround.It's been like this for ages.
It sometimes doesn't register as you killing the boss. I have 2 suspects:
1. Boss dying while you are trapped in the bubble thing
.
Dungeons
Vaults of Madness
Ulguna Soul-Reaver will now always grant credit for associated achievements when killed.
Looks like they fixed it on PTS 5.2.1:Dungeons
Vaults of Madness
Ulguna Soul-Reaver will now always grant credit for associated achievements when killed.
Looks like they fixed it on PTS 5.2.1:Dungeons
Vaults of Madness
Ulguna Soul-Reaver will now always grant credit for associated achievements when killed.
Nope... they sure didn't fix it... outright lie by ZoS... I just ran it and still, four years and the no death run achievement remains incomplete for both my wife and I. Countless hours wasted in this place.