Try making Senri for the main character (naturally Null to Fire). You can get stuff like Red Wall, Resist Physical, and Bufu on her.
What's the date in the game? Early on in the game, the fusion forecast usually has one special day before the end of a dungeon where you can make something to use on the boss/protect against what the boss can do. For this dungeon, that is:
4/28-Create Berith (Hanged 15), Mabufu added.
What I'd do is make Senri (Forneus + Orobas is one combo) so the main guy won't get nuked by the fire stuff. I'd get Bufu and Resist Physical myself, but what you put on her is up to you. If you want Chie to survive, you may want to just have her cast Bufu on the prince, then defend, or take the risk of doing another attack. Once the boss starts doing 'Burn to Ashes' though, either let Chie die or just have her defend continuously.
If you make Senri with Resist Physical, then you mainly only really have to worry about the main character dying from the fear/strong phys followup combo, which won't happen as often if you dizzy the prince, and won't at all once you kill him.
As a personal update, I believe I have found my final Alice build. Hooray for Alice #5.