ACR-2 sniper rifle
Select "Single Player" at the main menu. Select the "Start New Game" option, then press Up(2), Down, Left, Right(2), Down at the difficulty selection menu.
Easy kills
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.