Press start to pause the game and then enter the following.

Half Damage: x, x, y, y, left, left, right, right, y, y, x, x

Bulletproof: left, right, right, left, x, y, y, x

Invincibility: left, right, right, left, left, right, right, left, left, right, right, left, x, x

Infinite Ammo: x, y, x, y, x, y, x, y, left, y, x, y, x, y, x, y, x

One-Shot Kills: y, left, right, right, left, y, black, white, y, x, x

Turn M7 into Frag Launcher: black, white, right, right, left, left, right, right, white, black, x, y

Turn Crossbow into Frag Launcher: left, right, right, left, x, y, black, white, x, y, left, right

Big Weapons: x, x, x, x, y, y, y

Big Feet: right, right, right, left, right, left, right, left, left, left

Skip Cut-Scene: black, x, black, y, black, left, black, right, black, white, white, x, white, y, white, left, white, right, x

Watch End Cinema: white, black, x, x, left, black, black, x, x, white, x