Hidden difficulty setting:
Enter the options screen, and set the music to ''0A''. A fourth
difficulty setting will now be unlocked.
Destroy both of the level 7 Boss' arms without destroying
the Boss itself. Note: This is the large robot with arms that
launch themselves to attack. Catch the words "BONUS STAGE"
when they fall from the Boss. Then, destroy the Boss to start
a bonus level containing numerous power-ups.