After progression through the mission, you will take control of the Sniper Drone (WASP) and then there are two methods for this:
1. As soon as you gain control of your drone, mark your target, then go left and descend as low as you can go. You'll notice a group of 3 enemies very close to each other. You'll need to line two of them up with the one guard that is further away. The two far guards will line up perfectly, but there is one that moves around a little, and you will have to wait for him to walk in front of the others. The bullet must go through the head of the first one in order for it to travel all the way to the third guy.
2. For this method, mark the target, and take out the two guards near the truck. Now quickly go right, over the parking lot, and descend as much as possible! You should be able to line-up 3 guards on the lower walkway connected to the parking garage. One of the enemies will walk back and forth, so wait for them to all be in a row before you take the shot. If they are taking too long, you will have to follow orders and kill the two guards on overwatch, and then return to this location so that you don't fail the mission.