Super Tank - L1, L2, L1, Circle, Square, Circle, Circle, L2, L1, R2
Invincibility - L2, L1, R2, Circle, Square, Circle, Circle, L1, L2, L1
Bonus Level - L1, L2, L1, L2, R2, R1, Square, Circle, Square, Square

Double the strength of your shield:
When you are selecting a tank, press and hold down L1+R2+L2+R1 and Select. Hold the buttons until the level starts.