Free troops:
Put the pointer on the troop roster and hold Select. Then press O, Triangle, X, Square, Left, Right. Now your troops will fight for free.

All Units:
Put the pointer on the spare book on the left and hold Select. Then press R1, L1, L2, R2. Now your troop roster will contain every unit in the game.

Infinite magic points:
Put the pointer on the book of magic and hold Select. Then press Left, Up, Right, Down, L1, R1. Now whenever you cast a spell during a battle, no magic points will be deducted.