Level skip
Press [C] on controller two during game play.
Level select
Use the sound test selection on the options screen to play sounds 1, 1, 9, and 2 (in that order). A round selection screen will appear.
Ninety-nine ships
Enter the options screen and set continues to "1", difficulty to "Hard", ships to "2", and sound to "65". Press [A] to begin game play.
Ninety-nine bombs
Press [C], [A], [C], [A], [Start], [B] on controller two at the ship selection screen. A chime will confirm correct code entry.
Start at level 2
Hold [B] and press [Start] at the title screen.
Level 20 firepower
Enter the options screen and set continues to "2", difficulty to "Hard", ships to "3", and sound to "77". Press [B] on controller two during game play.