1. Start a new game or load an existing one.
2. At Title screen (Press Start to begin) wait until the developer’s logos stop showing up at the bottom of the screen.
3. Press Select at this time(while the logos are gone) and don't let go of the button until you get the Dev cheat menu.
4. With Select still pressed, Press Start to begin and don’t let go until the game loads.
5. Once you’re in-game, press Select to toggle the Devcheat menu on or off.
6. When the menu is on, press R1 for infinite health.
Method 2: Enter code
1. Start a new game or load an existing one.
2. Press Start to get to the title screen.
3. Enter the code: Up, Up, Down, Down, Left, Right, Left, Right, B, A.
4. Start the game. You should now be invincible.
Note: This code may not work on all versions of the game.