To unlock all levels, enter the following at the mission select screen:
Y, RB, Y, RB, X
To enter the folllowing codes, pause the game and hold down the Back button, Left trigger and Right trigger:
full life: LB(x2), RB, X, RB, Y
invincible Scott Mitchell: Y(x2), X, RB, X, LB
team invincible: X(x2), Y, RB, Y, LB
unlimited ammo: RB(x2), LB, X, LB, Y