Hold the back button and open up your menu. Upgrade your agent's skills to max. Go to spawn vehicles. Spawn 5 hatchbacks. Press X to pick them up and X again to lightly toss them into a pile. Go back to your menu and scroll down to spawn weapon. Get a grenade launcher. Shoot the middle of your car pile.
Alternate Way - Thanks Rista
No need to get the grenade launcher. Upgrade skills then spawn 5 close together and pick one up and throw it at the others...boom! Achievement.