Infinite Ammo:
Up, Up, Down, Down, Left, Right, Left, Right, R Trigger

Unlock Battle mode:
To unlock the Extra Battle mode, you must beat the game on both Arrange and Original modes. Also once you do this, a short cinema will show with a zombie of Brad. If you do not collect any items in the very beginning, when you go down the stairs in front of the Police Station, Brad's zombie is there. Kill him and he drops a key to the lockers in the Darkroom.