Find Battlegate 1 from the Summit on Olympus. By this point in your trophy/achievement hunting, you should have the Ultima Weapon keyblade unlocked. For this strategy to quickly and effectively work, target any initial enemy with your Ultimate Weapon shotlock. It’ll clear the group, earning you around 14,000 experience. When the next wave spawns, sling around a few Thundaga spells combined with its grand magic finisher before using your shotlock once more.
The method may take a moment for you to grow accustomed to, but once you have it down, you’ll fly through each battle.