Story Related, Act II (Post Game)
*** Spoiler - click to reveal *** the battle you been waiting for
Mordegon, he will confuse you and when he uses the purple mist you will be unable to heal, if you use bounce you'll be protected from most attacks besides his normal staff smacks which isn't too much to deal with. Other than that it's a simple fight.
The achievement will pop once you regain control of the main character