Fight against Exdeath while making sure to equip your character with Magic Pot (not auto). Max out all of Exdeath's stats since you earn more experience this way and make sure to equip an HP attack that can execute faster than his Almagest attack the only exceptions being Jehct and Ultimecia (whose HP attacks all require charge time. A level 100 Exdeath tends to abuse his teleporting abilitiy, which can be easily guarded against and the only HP attacks he will use are Almagest and Delta Ray and Grand Cross, which are all easily evaded. The trick is you want Exdeath to break you until his bravery amasses to 9999 damage points, then use Magic Pot (remember that it only activates when your character is not in break mode). Once your character has 9999 break points, you can easily finish off Exdeath while he uses Almagest or by blocking his teleporting ability and countering (every character has at least one attack that will provoke Exdeath into using teleportation). If successful, your character will earn over 100,000 experience points and on special calender days over 300,000.