He releases all remaining adds and spawns blue orbs that fly around, if they reach anyone they explode for knockback and high damage. He also casts Magma Jets, this leaves a line of fire the direction the boss is facing so you should make sure you aim the boss well. There is currently a bug with this ability that means if the fire goes out of the entrance door the boss despawns.
Best to save bloodlust for this phase and just zerg it down, offtank the 2 big adds (if you have lots of little adds still left to spawn aswell.. this could be painful).