Make a pause in game then HOLD L+R and then enter the following code a sound will make you know if it's good ...

Invincibility: B, X, B, Down.
Perfect mode: X, Up, Y, Down.
All attack upgrades: Down, Up, Y, B.
Full health: B, B, X, X.
Unlimited missile weapons: B, B, Up, X.
Always devastating: Y, Down, Y, Up.
Target indicator: Up, X, Down, B.
1,000 experience points, Aragorn: Up, B, Y, A.
1,000 experience points, Frodo: Up, Y, Up, Down.
1,000 experience points, Legolas: A, Y, Up, A.
1,000 experience points, Gandalf: X, Y, Up, Down.
1,000 experience points, Gimli: X, X, Y, A.
1,000 experience points, Sam: Y, A, Down, A.

Unlock Four-Hit Combo For Aragorn
Pause the game and hold both triggers. Then press UP, X, Y, DOWN.

To activate cheats, hold down R and L triggers.
Infinite Re-Spawns for Co-op: All characters B,X,up,B
All Experience you get, your buddy gets: Co-op down,A,A,A
All Health you get, your buddy gets: Co-op Y,up,X,X