Invincibility
Pause the game and hit the following buttons:
TRIANGLE, TRIANGLE, CROSS, CROSS, CIRCLE, SQUARE, CIRCLE, SQUARE, DOWN, LEFT
A sound should be heard to indicate your new status.

Select Any Level
When at the title screen, hit the following buttons:
SQUARE (5 Times), TRIANGLE (5 Times), CIRCLE (3 Times)

Hints
You must firstly eliminate all of the enemy craft in wave no.1 on stage 2.

When the wave 2 enemies start to approach, shoot only the first ship and then AVOID all of the others. Once this has been done all of the enemies will explode and you will be warped to a new hidden level.