Enter the following codes during the "Difficulty Select" screen to get the desired results:
Starship Level Complete
Left, Left, Right, Circle Circle, Right, Right, Left.
Alaska Level Complete
R1, Circle, R1, Square, Left, Left, Right, Left.
Deep Amazon Level Complete
Left, Right, Left, Left, Right, R1, R2, Circle.
Earth Level Complete
R2, R1, L1, L2, Square, Circle, Square, Circle.
Mid Atlantic Level Complete
Circle, Square, Circle, Square, Right, Left, Left, Right.
All Mini-Cons: R2, R1, LEFT, RIGHT, LEFT, RIGHT, CIRCLE, SQUARE
One Hit Kills
While playing, pause the game and press SQUARE, CIRCLE, SQUARE, CIRCLE, L1, L1, L2, L1. You will hear "Code Accepted" when entered correctly.