A good place to get this achievement is Level 3 - A New World ( using Wyatt Timeline )
After the chainsaw section you will be in the back of the old guys car, push X to leave and collect the armour in front of the car.
Stealth through the begining to the first building and go up the stairs then pick the lock to enter the building.
Climb the ladder inside the left room after you enter.
Climb another ladder to find the Marksman's rifle and some ammo.
After your first shot the alarm will be raised and a drone will launch to your area.
Take out the drone then look down the scope.
There should be 4-5 enemies that can be easily taken out.
If you don't headshot them they may require more than one bullet.