Another easy spot to do this is in Chapter 3 once you get the objective, "Raise checkpoint platform." The old man will shoot the guard answering the door, alerting the men inside. However, the remaining soldiers will not leave the room. Stand near the doorway, preferably just outside, and one of the Nazis will pull out a grenade (he'll also say he's throwing a grenade in German). Shoot him before he lobs it and the grenade will drop, killing anyone nearby. Reload the checkpoint until you have five kills.
Edit: Wow, Paul Doyle literally beat me to submitting a solution by a minute. But yes, that is the spot I will be using to farm the achievement once I complete the campaign.