The first level you can get this on is Episode 2's "Spaceport" (though you can also get this earlier by using cheats). You can find it near the red key. There is a well lit section which will lower you down if you press
and leads to the Shrinker (and a couple other goodies). You can then use it to shrink practically any enemy (outside of bosses).
The easiest way to get this is to shrink an enemy with it, stomp on them and kill yourself with an explosive. Rewind to the moment you began stepping on the enemy and repeat until this trophy unlocks.