Sniper location:During the caves level of the third mission, there is a hidden pathway behind the waterfall that leads to a sniping position overlooking Al-din's camp.
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 hint:
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.