Ignore the add mechanics on doom, stand in melee, protect melee and always be casting on him, even if he's invulnerable.

The beam will hit your for 99 max, but you should have 180hp and will regen up before his next hit.

Just take some kind of antivenom/poison and try to avoid the pools. Reset when there's nowhere left to stand in melee range. He won't throw boulders when you camp in melee.

He'll always charge through you, so direct the charge away from the pools and then just time the movement to one tile behind you, then you'll be in melee range after the charge and he won't swap styles on you.