Which part?How to defeated bos in festival of greed?
If it's the 1st fight with both Arwin and Mammon, just remove collar and then in the next fight have Aura use the ability to restore her collar, and then remove collar again.
All you gotta do is have a little bit of Perika saved.
If you're having trouble with the last fight, just drain mana when you need it and keep yourself healed, shouldn't be that hard.