Instructions
Equip a member of your party with the yellow plate. This object will absorb yellow elemental attacks.
Cast the "Diminish" spell on the battlefield. This will halve elemental damage.
Equip items such as earthquake traps that prevent Crisophinx from casting "Sprain," which would reduce your party's attack power.
Spam Crisophinx with "Imbecile" and "Weaken" to lower his magic and defensive stats.
Attack this boss with your party members now that you are ready for the heart of the battle. The fight is lengthy, as Criosphinx has 13,000 hit points. Heal your party members as necessary during the fight.