During Level 13, hold onto a mercury bow that you get in the stage. Make sure to have some ammo. While fighting Shinitama, go up to Griffan's force field. If he is not standing near the force field, hit one of the switches and return. After he is near the force field, shoot him. If done correctly, he will get hit and come through the force field at you. Just hit him a couple times to win. Note: You cannot get the "Didn't kill Griffan" last Boss battle if this trick is used.