Is you use Rolling if he does Fireburst and so on you will get no Damage at all.
Fight with all you have, go a away for little Regeneration and fight him again.
Hes a friend so he have 3 Lives like yourself.
Rollings in 3rd Modus is the Key.
I had no really Problem doeing the Boss with my Dual Melee Mage at Level 21.
Okay i needed One Life at all :-D

Have fun. Otherwise go in StoryModus to lower your difficulty.
I prefer normal Modus, maybe its get a little more difficulty in future game too.