This is a bronze achievement worth ten gamerscore

To get this achievement you must Kill 4 enemies with a single smart grenade. this can most easily be done on mission four Fission. After you destroy the big walking tank you will go into a building, have to slide down an elevator shaft and come out in a laboratory. Make your way through the laboratory. The spot for the grenade kill is in the next area. Once you leave the laboratory you must sprint-dash to the left immediately! There are 5 soldiers spawning on a small platform. Position yourself on the stairs that lead up to this platform and throw the threat grenade into the air. Because this platform is so small, all enemies will be standing close together. And because you stand on the stairs it will stop the enemies from going down the platform. The smart grenade will kill 4-5 enemies and it works every time. Just be sure that you get to the stairs quick enough before one of the soldiers gets away.
Luckily, there is also a checkpoint when you enter this area. So reload the checkpoint if you need to try again and you'll restart in exactly the same spot.