Make sure Final Chapter is done. You may be able to do this with Aqua and terra as-well, I'm not sure -
Equip a deck of:
Head to the reactor in Radiant Garden. As soon as you see the Tank Topplers use Mega Flare to take care of them. Then as soon as you regain control over Ven use Mine Square then Thundaga. Seeing as this takes about 1 minute to do. In 15 minute you will have 30,000 exp. Which is an easy way to train for unlocking Ultima or the secret bosses.