This achievement can come through natural progression of the campaign, but it is also easily boosted during
Act 2 Chapter 3: Plan B.
You'll come to a section where they ask you to choose between 2 paths (left or right), and you'll want to choose left. After spawning, you'll notice some enemies in front of you with some bales of hay being held by a wooden gate. Shoot this wood to release the hay and kill the enemies. Now, look across the river/creek and do the same on the other side to kill a total of 4-5 enemies. Now,
Reload Last Checkpoint and repeat this process until the achievement unlocks. Take turns if you're playing in co-op!