Phantom Mane Resetting

[Redsquirl]

Have been trying to kill this boss for about an hour now and it seems impossible. Even if I manage to get lucky and not be grabbed and pounded for 70% of my HP every time or obliterated by his AoE circles that seem to be about 10 meters wider than they appear, when he gets low he just resets to full health. Is this supposed to happen? I am not leaving the room, I am right next to him in the middle when he resets and it's usually after one of the grabs.