I found this was easy to do on act 3.
As you drive a quad bike through a tunnel you come a pool of water with electricity. You have to stop time and jump it on the quad. When you land you get a loading screen.
Once the game has loaded you have a very short way to drive to the end of the tunnel. As you get near to the end of the tunnel you will see 3 enemies walking away from you.
Get off the quad, get your shotgun out and sneak up as close as you can until they say "weapons ready" or "weapons hot". As soon as they do freeze time and put a few shots in each. They should all die and the achievement should pop.
If it doesn't you are right at a check point so just let the survivor kill you and do a quick load and you will be right there with the 3 enemies ready to try again.
Hope this helps.