Summoner
Quick spell
Cast a spell, press Triangle to display the pause menu, then select the "Inventory" option. Exit the screen to resume the game with your spell being cast without a delay.
Reduce AP cost of long chains
Get a "Chain 4" or higher attack, then quickly press R2, L2 (or L2, R2) to switch to another character and then back to the original character. If done correctly, your chain number will reset to 1 without slowing your attack.
Hidden FMV sequence
Select the "Credits" option from the main menu,
then press X while the credits are being displayed