At the main menu, hold Left for three seconds, then press Right. Next, press Circle, Triangle. If you entered the code correctly, the "Option" selection will turn red. Then, enter the option menu to change the difficulty setting to extra hard.
Play as the enemy
At the main menu, press Right, Down, Down/Right, Down, Down/Left, Left, Triangle. If you entered the code correctly, the "Game Start" option will turn red. Then, select "Game Start" to access a menu that will allow all bosses and some enemies to be selected for game play.
Versus mode
At the main menu, press Down, Circle, Right, Square, Up, X, Left, Triangle. If you entered the code correctly, you will hear a sound. Then a versus mode option will be available.