The easiest way to do this is on Chapter VIII Checkpoint 3. Kill the two guys running down the stairs on the left and right first then just shoot the grenades that the guy up on top in the middle shoots at you. He has an unlimited supply of the grenades so you can keep shooting them. I did this to get the achievement and to finish the "Grind" for that which is to shoot 200 Airborne Grenades, just reload your last checkpoint when you runout of ammo. Just make sure that you have Hard Lock turned on and you should be able to get this achievement easy.