itch.io

Get him to like half hp or a bit above then use divine retribution in celestian form

Truthfully the way managed was I kept fighting him til he ran out of mp to keep healing. You can keep switching between Celestian form and normal as long as your patient enough. My first fight with him took 20 minutes because I struggled with the healing too

It's a matter of wearing him down. He'll run out of MP eventually. Although, if you want to speed up the process, there's a specific skill tied to a piece of clothing that allows you to drain MP from opponents. Or, if you have enough raw damage, you can stop time right as he's about to heal, and kill him like that.