Play through the level " Military base " 3/4 the ways through.
And you'll get to a room with a lot of boxes and explosive canisters, but no enemys. The room for the achievement is the next one.
Keep heading down the hall and you'll hit a check point, now is the time for the killing.
Straight ahead will be a barrel, shoot it them immediately then throw a grenade, Hit Y and run up the stairs to your right, if timed right there will be a group of 5 or so people you can get all at once with one grenade launcher.
This ones hard to get but keep trying.