The best place I found for this is Mombasa Streets' Rally Point 5 (Sniper Rifle).
As soon as you go out the first big door on the way to the waypoint, there will be at least 5 sleeping grunts in the corridor, as well as one brute. Kill the first few sleeping grunts, and then go to the right side and meleé the brute from behind (he's carrying a carbine on Easy difficulty). Then just meleé the others.
Easy achievement :)