Mark the target, then take out the two guards near the truck. Quickly go to right over the parking lot and descend as much as possible. If done correctly, you should be able to line up three guards on the lower walkway connected to the parking garage. One of the enemies will walk back and forth. Wait for them to get aligned in a row before taking the shot. If they are taking too long, you will have to follow orders and kill the two guards on overwatch so that you do not fail the mission before returning to this location.