I struggled with multiple other solutions I found online, but stumbled across something while going for the other perks.
You will need to have the scope attachment. This can be found in Chapter 11 (U-Boat). After the first room with the commander, you'll go upstairs and through a series of doors until you reach a hallway. To the left is a locked door (and LKW charger), and to the right are a few enemies in the living quarters. Take care of them, and head back to the locked door. You will need to hotwire it open, then turn and look down on the floor to the left. There is a grate you can cut here with your LKW, and it leads to a secret room with the scope attachment on the bed.
Load up Chapter 16, the final mission, and once you've swam to the compound, make sure you load up on armor. Even on the lowest difficulty, you'll want to make sure you are protected. There's plenty of gear in the catacombs, and then you'll go up, kill the robo-dog, and right after him you'll hit a checkpoint. This hallway is similar to the lobby shootout in The Matrix — lots of pillars and guys coming out of everywhere (regular soldiers, the heavy armor shotgun guys, and I believe even one rocket soldier). Clear them out however you'd like, but DO NOT go to the end of the hallway until you've cleared all the soldiers. Let the grunts come to you.
Once they're cleared out, make sure your LKW is fully charged — there are a few charge stations in the lobby — and go into scope mode. Head down the lobby hallway and look to the left. One of the large robots/mechs will be standing there, and he'll most likely turn to you and start firing — good thing you loaded up on armor. Make sure you line up your LKW shot, with all of the red lights at the bottom of the scope illuminated, and fire away. It will discharge the entire clip in one shot, and if you had it lined up, will be a direct hit. The achievement will pop then.