Very simple method to obtaining this achievement, as well as the earliest you can get it.
While making your way through the game, during Episode 2 - Mission 1 (Spaceport), directly before the red keycard lock opening the way to the level’s exit is a brighter section of flooring. “Use” the floor while standing on it to be lowered into a secret area, around the corridor to the left will be a Shrinker. Grab it and reach the level’s exit.
Upon reaching the subsequent level (Incubator), proceed forward through the two doors to reach a large darkened room. Immediately save the game (a door will open directly in front of you revealing the enemy you’ll be using for the achievement). Now just shrink and stomp, reloading your save after each kill and you’ll have the achievement in less than 10 mins.