If you're like me and finished the game with all upgrades, and don't want to reset stats
There is still hope.
I made a quick video capture of how I did this, on the easiest difficulty with having all upgrades.
When you enter area run all the way to the back left but don't enter into the elevator room.
Group the guys up and flash the shit out of em. Get some distance and spray them with the shotty. Hope this helps you out with this annoying achievement