ABADOX

Invincibility:
On the title screen press A, A, Up, B, B, Down, A, B and start and you're
invincible.

Stage select:
On the credits screen, press Left, Up, Right, Down, Up, Right, Left, Down
and then press the following:

Up   - Stage 2    Right  - Stage 5
Left - Stage 3    Select - Stage 6
Down - Stage 4    B      - Stage 7