First unlock the cheat selection by enter:
Down, X, up, Y, down, down, up, up
Then use this code to get a shotgun:
KKsh0tgun (its a zero not an O)
Then use the cheat for 999 ammunition:
KK 999 mun
Then reload you're shotgun.

Do this repeatedly and you'll have lots of ammo!

At the main menu hold left trigger and right trigger, then enter, down, x, up, y, down, down, up, up to open the cheats menu.

God Mode: 8wonder
All Bonuses Completed: KKmuseum
One-Hit Kills: GrosBras
999 Ammunition: KK 999 mun
Machine Gun: KKcapone
Revolver: KKtigun
Shotgun: KKsh0tgun (contains number 0)
Level Select: KKstOry
Unlimited Spears: lance 1nf (contains number 1)
Sniper Rifle: KKsn1per (contains number 1)

Hold L+R and press Down, X , Up, Y, Down,Down,up,up (at the main menu).Release L+R.This opens the cheat menu.

Invincibility:
Activate Cheat Mode, then enter 8wonder

1 hit KO:
Enter Cheat mode and enter GrosBras

999 Ammo:
KK 999 mun

Infinite Spears:
Lance 1nf

Machine Gun:
KKcapone

Level Secect:
KKst0ry Notice there is a zero not an O

Bonuses Completed:
KKmuseum

Revolver:
KKtigun

Shotgun:
KKsh0tgun once again there is a zero not an O

Sniper Rifle:
KKsn1per