Pause the game during battle and enter the codes:
|Up, Up, Down, Down, Left, Right, Left, Right, L1, R1||Rex armor gauge will completely fill. Only works ONCE. If you use it and save, you can never use it again.|
|Up, Up, Down, Down, Left, Right, Left, Right, R1, L1||Rex armor gauge will empty.|
Thanks to Revolution reader elderling!