If the game seems too hard and you don't mind cheating, enter any of the following keypad combinations at the Press Start screen:

To access all levels, type the following keypad combination: 

L1, R1, L2, R2, triangle, cross, circle, square, square, circle, cross, triangle. This cheat code allows you to access all levels when you get back to the Start Mission screen.

To access all types of player ships, weapons, and upgrades, press L1, R1, L2, R2, up, down, right, left, left, right, down, up.