How do you defeat the pride demon? He just keeps reviving himself
These tricks of theirs with restoration depend on magic power (like the blue scale of our blonde, on the top left of the screen). If you find a way to influence this parameter of theirs, you will make their task more difficult. Hint: this can be done with the help of skills that are obtained through the active use of artifacts in our blonde's work with monsters.
The demons are not that hard to defeat, really. You just need to stay alive and keep hitting them until their regeneration ability finally gives out. Use your potions and the Celestian form when it becomes available, and have patience.
Edit: Make sure you are at maximum level before you face them, too. You should be at or near it anyway by this point in the story.