These are the tiny green dots in the game that show up after killing something. If you hold RT you see a fairly large circle. You can hold RT the whole time you are playing and any that enter the circle you suck up automatically. Also with out holding this if you run into a patch of them or get close enough you will suck them up. Since these reload your green bar at the bottom middle I just hold RT the whole time I am playing. If you do that this will unlock after a while, depends how good you are and how many levels you beat. You will eventually get this playing normally. This is cumulative so each game you play counts toward this. The xbox one achievement tracker seems to work for this game so check you status often if you are set on getting this.