The final boss is
. All you need to do is not get hit by his ultimate attack. You'll know when he's about to do it when he stands still and disappears into a cloud of smoke. Be careful, as he'll teleport to you even if he's halfway across the map and smack you.
Luckily, if you do this on the easiest difficulty and have all the modifiers on (double damage, double health, infinite ultimates, etc), he won't even get a chance to get his ultimate off if you stay aggressive. I used Duke and his health bar just melted all three phases.