In the mission select, move the cursor to an area that isn't a mission. Hold the Start button and press A, B, C, B, A, A, B, C, B, A, B. After the code ring sounds, move the cursor to a mission area and press (and hold) the following to fight the boss of that area.
With any of these options, if you also hold the start button, you'll be invincible.
Fighter Ace - A.
Super Carrier - B.
Last Fighter -C.