Flintlock: The Siege of Dawn

Upon reaching Mile's End Barrier shortly into the story you'll find a chest with a grenade in it, and then be attacked by a large host of weak zombies which are tied to a sort of mini-boss whose health is tied to how many of them are left alive. You'll fight a few of these types of enemies throughout the game. These each give you a good chance to unlock this achievement. Let the enemies swarm you and then drop the grenade or shoot one of the barrels to quickly take out a large group of them at once. If you have your ultimate attack charged at this time you could also use + to unleash that and kill a group of them.