Sonic's Secrets: AirDiver.

Level Select On the Mission Select screen (see right), move the cross-hair to a place on the map that's not an area. Press & HOLD Start then press: A, B, C, B, A, A, B, C, B, A, B. Release Start and you will hear a soft ping confirming the code has been accepted. Now move the cross-hair to the area you
wish to attack and press & HOLD: A to square off against the Fighter Ace, B to fight the Super Carrier, C to battle the Last Fighter. NOTE: You must hold the button until the area begins.

If you want to be invincible then enter the Level Select code as above, then press and HOLD Start while pressing the button for the level you wish to tackle:

Start + A to square off against the Fighter Ace,
Start + B to fight the Super Carrier,
Start + C to battle the Last Fighter.