You need to be more mobile and you can avoid all his damage. EVERY slam of his arms is easily avoidable. His tell is he puts a plague pool down before he slams. You have a second or so to move out of it.
The trick is to be proactive and not reactive to the abilities. What I mean is do not just try and spam your damage skill in between his slams but to just pop off 1,2 or 3 damage abilities and start moving being prepared for his next attack.
I like to stay in the lower right hand of the screen that way you can avoid his acid breath attack. When he lays down all the plague pools I do not attack at all. I strictly focus on moving out of the pools to whatever spot currently is open.
A lot of people get hung up with trying to put out too much damage and I believe all deaths in P3 are because people don't watch the tells.