To activate Cheat Mode, at the main menu hold LB + RB + LT + RT and press Down, X, Up, Y, Down, Down, Up, Up. Release L + R.
Now use these cheat codes:
Invincibility: 8wonder.
One Hit Kills: GrosBras
999 Ammo: KK 999 mun
Infinite Spears: lance 1nf
Machine Gun: KKcapone
Level Select: KKst0ry
Bonuses Complete: KKmuseum
Revolver: KKtigun
Shotgun: KKsh0tgun
Sniper Rifle: KKsn1per