When you are in the level where you have to go into the caves, reach the base that is halfway in water. There is a waterfall. Go through there, making sure you have a sniper rifle or an assault rifle with a rifle scope. Continue until you see a bar like a jail cell door. Go to your scope on your sniper or gun. You will see little people walking. You can snipe them all and they cannot hurt you. Note: A frag grenade will not work.