Abadox (Nintendo (NES))

Invincibility:
Press A(2), Up, B(2), Down, A, B, Start at the credits screen.

View end game sequence:
Press A(2), Up, B(2), Down, A, B(2), Start at when the phrase "Press Start" appears.

Level select:
Press Left, Up, Right, Down, Up, Right, Left, Down and one of the following buttons at the credits screen.

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

Hint: Defeating the robot Boss:
Position yourself in the safe location near where the beams are shooting from the left side of the screen.

Hint: Defeating the "Eyeball":
Position yourself in the safe location just to the left of the "Eyeball".