The super easy way:
- Change your difficulty to “story” (optional)
- Travel to Byzantium on Terra 2
- equip any weapon that has the blue lightning bolt icon
- save your game
- shoot Gardener robots, ad drones, security bots, or any other robots you see
- reload your game and repeat
NOTE: The above solution is exactly how I earned the achievement, but some people have issues. If the achievement does not seem to be progressing when you reload your save, try this: after you shoot the bots, save your game to a DIFFERENT slot, then reload your first save. Keep repeating this because saving your game seems to progress other achievements that normally don't.
From Cobb AUS: "Worked like a charm! Changed the difficulty every time i reloaded the save and it counted towards it."
From ChooseMyFate: "The tracker doesn't lock in your kills till you save in another slot. If you go to your achievements tracker after getting kills you'll notice that the bar is a different color gray for the kills you got. Save in another slot. Go to the tracker again and you'll notice it "locked in" the kills. Load the save before you killed the robots. Rinse and Repeat!"