Ive noticed in this fight, Tusk will continually have collision issues, if he comes out of the water at all he begins to regenerate health, he can sometimes have a collision issue where he ends up on top of the rocks and freezes - regenerates all his health and then spawns in the middle again. when this occurs the boss is effectively reset but you are not and limited to your heals etc that you have used.
I’m attempting to get a video of the interaction and will add it.