This code is very simple, and it works with every player in the game except Purge. All you do is start the arcade mode, then when you fight, pause and do as follows:
1. Go to the disables menu
2. Turn off shields
3. Go back to the game and do nothing but fire!

I was amazed myself when this turned out working. The only thing you have to watch out for is when Mutilater (the bad guy) shoots into the water. You have to get out quickly and keep firing, because there are parahnas in there that will eat you.