There are a few steps to unlocking this achievement.
1. On your first visit to the Colosseum, talk to Phil. He'll ask you to move a large boulder.
2. After failing to move the bolder, talk to Phil again for a scene.
3. Talk to Phil until you get an option to break barrels. Finish both time trials.
3.5 If you are struggling to finish in time, remember to attack the middle barrel when they are in groups. Also, if you have any combo extenders on, turn them off.
4. Try to leave the world through the big gate outside for a scene. You'll get an entry pass.
5. Beat the first challenge. It's very easy. The last match with Cloud's outcome (win/lose) doesn't matter.
6. Save when you're returned to the lobby, then go back in & fight Cerberus.
7. If you haven't locked the keyhole in Traverse Town, do that now. If you have, leave the world to be told there is a new cup available at the Colosseum, and re-enter to try the Phil cup. (
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8. Make it through all the waves of the Phil cup to unlock the Gravity Spell & This achievement.