This must be done in one playthrough, and kills are not saved until you see the "TR" symbol in the bottom right. So you can't farm this by reloading checkpoints or dying. There are somethings you can do to make this go faster, however.
--Buy the long-range upgrade early. When this, you can press [cn_RB] for a shot that travels further. It's still only medium range, but you'll have more targets available.
--upgrade your ammo capacity (skill points) and shotgun damage (salvage). There's some parts of the game with lots of targets, but no shotgun ammo. Conserve ammo.
--avoid finishers([cn_Y] near downed enemy) (once you have 15 for that achievement, anyway). Shoot them with your shotgun instead for extra kills.