The wave I was mentioning can only be cast 2 seconds after Moosi awakes. If you’re dying that early into the fight, then maybe it’s all his skeleton archers “shotgunning” you with arrows and dealing a huge burst of physical damage? That can happen if you use a displacement rune to quickly teleport into him to start the fight: it causes all the minions to fire at you in sync. If that’s the case, then starting the fight by evading into him might help.
Edit: in general it’s hard to tell exactly the cause of death in Grim Dawn, but there are 2 things that can help with that:
1 - recording video of your gameplay, and after you die checking the footage frame by frame
2 - Install DPYes. It shows you what types of damage you are being dealt, and also your DA which helps understand if you are being crit or not.