To unlock this achievement, you'll have to play and clear levels 31-50 in the single player mode. Your clear rank doesn't matter, you just have to successfully clear all of them. You don't have to collect a single star for this achievement, just get the required number of Leedmees to the exit.
Make sure to make all of your movements as smooth and deliberate as possible. Moving too fast will only cause you to fling your Leedmees across the screen instantly killing them.