First, fast travel to any location and purchase the RPG-7
for 160,000 K Rupees.
Next, go to Rochan Brick Co. Storage
(X: 440.4, Y: 542.5). If you already have it captured, do the "Outpost Master" quest from inside. From the outside of the base, you'll just want to run in, shoot someone and let the enemies call for reinforcements. When the reinforcements are called, follow the road downhill from the base. Once the reinforcements are called in, two trucks
will spawn near the bottom of the hill not far from the base. The first truck will have a turret, but only 3 enemies, so let it pass. The second truck will have 4 enemies, so use your RPG-7 and shoot it once to cause an explosion and kill all 4 enemies, in order to unlock the achievement.