Have your party at least at level 50 and have the Curaja spell. In the battle, there is a constant HP sap in the area that cannot be stopped in any way. Have one character use Curaja always, and the other two attack the Esper. Dispel the Boss immediately, give as many characters as you can a Black Belt, and stop his Disable attacks. Make one of your characters have Ensna as one the first Gambits. When Foolbars join the battle, use your strongest Fire-type spell against them. The Esper likes to use Invert to quickly kill your characters. It resembles an instant-death spell and in combo with the HP sap, and can kill your people very fast. Revive them immediately and use some Quickenings if your first team gets knocked out.