ACR-2 sniper rifle:
To unlock the ACR-2 sniper rifle, start a new game in Single-Player mode and at the difficulty selection menu enter: up, up, down, left, right, right, down.
Easy kills:
In the level where you go into the caves, reach the base that is halfway in water. There will be a waterfall. Go through it. Have a sniper rifle or an assault rifle with a rifle scope equipped. 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 damage you.