Go to the very first mission and start. This cheat may work with the rocket launcher, UZI, 9mm, and silencer codes. Go to the portable. stand close by and watch were the guy runs when he comes out. when he comes back, go in, kill him and use the face mask. Come out and take out a gun and head to where the guy went before. Make sure the second guard straight head, near the other portables is in front of you. Shoot him down and run to the electric panel ahead. You can blow it up if you want (depending on your objectives). It may take a few tries, but the guard should come back as a half dead person (ghost)and shoot at you. You will be affected, but not him.