When on the desert level of the combat arena activate one of the jetpack flyers and fly around till you find a mountain type piece of land that is NOT connected to any other piece of land, that you can fly to the top of it and land your jet pack when up there hit the crouch mode and 9 out of 10 times you will not be seen.
If you have a good sniper or rifle with a scope you can get head shots which kill with one shot.
Oh yeah almost forgot if you find the right sniping point then it will say Dirty Sniping Point.