On the mission Atlas after the shooting range you will go to a grenade range. You need to score 1700 points to unlock this trophy/achievement. A easy way to get this quickly is to equip the Threat grenade and Smart grenade. You will throw Threat grenades to show hidden enemies and shoot them through the walls. Throw Smart grenades to take out drones and moving targets. Smart grenades automatically search their targets so throw them anywhere in the air. You can ignore all the other grenades as their not really needed. Just be sure that you stand next to the grenade refill station to refresh your inventory.