Enter this cheat inside cheat option in game settings.

Circle, L2, down, R2, X.

First go to the options menu, and choose game settings en than select ' cheats ' there you can enter the following cheats:

Unlimited ammo:
L1, L2, X, R2, R1

Invincibility:
up, square, circle, X, down

grenades as bullets:
X, circle, square, L2, R2

Kill enemy's whit one shot:
circle, L2, down, R1, X

Unlock al bonus:
right, R1, L1, down, circle