I personally get bored of reloading checkpoints again and again, like some websites suggest to do. Instead, here's a way to do it if you want to unlock it without reloading checkpoints.
• Near where Duke meets you (his forward camp by the Bandit Warehouse) You can spot a large group of mutants to the right hand side of the bridge (facing away from Dukes perch, towards the Terminal)
• It's possible (if you're extremly lucky) to get some inside the terminal too, but it's really about luck where and when the mutants spawn) and if any are currently annoying you whilst up close and personal.
• When you've restarted the generator, take the light up to the roof.
Multiple enemies will soawn and start fighting one another when you restore power to the lift in the barn. It's possible to snipe enemies whilst standing in the lift.
• Before then, you can boost using the wolves. The pack you first meet after the first appearance of the Bear. (after the first Pirate camp) You can get four or five kills, however, the rest of the pack do charge you, so it isn't the ideal spot.
• Complete the objective that allows you to get the car and then ignore the lighthouse objective. Make your way back towards the Aurora, instead look for a group of tents to the left of the road.
If you go to the cliff edge and move backwards towards the Aurora, you'll notice lots of Mutants around a dilapidated pier. Snipe them all and then move on. (This is a good spot to reload checkpoint using Quick save if you're that way inclined)
• Drive down from the aurora, towards the airfield, and stop at the edge of the parked cars. There are mounds for Nosalis' nearby, climb a car roof and crouch for better aim. You can normally pick a couple off before they run off. If they do. Climb the hillside to your right and there is a second group you can get three kills from.
• From here go to the sniper point that Anna normally tells you about and take everyone out as you see fit. Kill the sniper and go to his perch for an awesome Sniper and pick off the support that come (10kills). This can also be done with a Kalashnikov and a 4x sniper scope. (This is a second good place to save and reload checkpoints).
• Just down the road is a bandit camp that leads off by zipline to another camp just to the North. With your Sniper, it's possible to get at least four long range kills, as these particular bandits are pretty c
dumb. The camp to the north is a close quarters inside deal, and isn't really helpful for this particular trophy.