You need a party with Koromaru, Junpei, and and a healer that isn't Yukari or Mitsuru. That should lower the damage of the fire spells. Don't use Magic Mirrors cause the boss tends to spam Allmighty spells if you have it cast on your party. If you have a party member with High Critical hit rate attcks (Shinji or Aegis) then bring them in cause a All Out Attack will do serious damage.

This boss is more about luck than anything I'm afraid...