At the very start after that first house if you make your way up the road you will find some abandoned cars (past the cop car a ways). In one of the cars is a world mission to find the hunter house. The house is pretty much straight North of there along the water (look at your map and mark it). In the house is a pretty good rifle (in front door room) with a scope (1-4x in small bedroom). Now go back to the cars and follow the road into the field and you will see a church ahead. There are 3 or 4 dog machines there. If you crouch and sneak in use the rifle to shoot the energy pack at the top of their backs it is a ONE SHOT KILL. the first kill will attract the others so keep your distance and hide and pick off 2 more and the achievement will pop. If you screw up just keep heading down the road to other places they have several dog machines as well and repeat the one shot kill. Hope this helps. You will find several 4-8x scopes as you make your way further into game which will make this easier if you want to wait till you find one. The dog machines are your best bet for the achievement since they are ALWAYS one shot kills to the battery pack on their backs.