Very simple. Just kill 100 stewards in the Good Intentions level (you DON'T need to kill every Steward you meet despite the achievement description). Stewards are basically the robots that will be attacking you throughout the level (rectangular with the changing colors on the front).
Don't focus on getting this achievement as you should have it by the time you finish, however, if for some reason you have made it to the end (going back to the big circular room with the different pathways where you started), you will have an opportunity to get the achievement when the Stewards infinitely spawn.
Basically when you are at the end, back in the circular room, the pop-up guide will tell you to throw a charged cube at the target above the Steward's entrance. Don't do it. Just keep shooting the Stewards with Eve's laser hand until the achievement pops. Then go ahead and close the entrances.