Boss: Emperor of Fire
Attacks: Blow Crusher, Hellfire, Restoration, Melt Unit, Act Unit, EX Summon Phoenix
For this battle have Atsuma and Raigar in the front two corners (closest to him) and Lord Onyx and Yuki in the back two. This way he can only attack two people at one time. Then when the light is off, attack him with everyone (use Atsuma's EX last as he'll do the most damage) and heal when needed. If you keep them like this and you attack with your strongest attacks and EX attacks when they're available then you'll beat him easily.