Relatively simple achievement. Once your mining mech falls down the hole and you are back on foot, you meet the creepers, small crawling creatures. To get this achievement, you have to stomp on them with your foot, not melee them with your arm.
What I found is that if they are attacking, you will melee them with your arm and it won't count. They have to be sitting docile (not attacking) and when you push RS, you'll stomp on them and get a kill for the achievement.
Kills carry over between checkpoint restarts, so if you hit a part right after a checkpoint where there are a batch of creepers, try and stomp as many as possible and then reload the checkpoint.
Credit given to Teamwrk for his comments - "perfect location to net this achievement during the first part of the "Water Supplies" mission (after you return the power cells to Bastion). You walk into a room full of creepers with a spawning nest at the bottom of the room."
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