Start a game and go to the attack-craft select screen. Highlight your craft and press Button C. Hold down the top L and R buttons. While holding L and R, press and hold Buttons B and C. Continue to hold all four buttons and press the Stop button. This takes you to the Exit Game option. At this screen, continue to hold the four buttons and push Down on the directional pad. You'll hear a confirming sound. Do this for each craft you wish to make invincible.