This is located on Mission 5: Siwa Oasis
At the end of the mission, you will need to climb some scaffolding, then spot and assassinate an officer. Once they stop talking and split up, climb down the scaffolding and follow the target, which can be any of the three characters.
If you keep following him for about 1-2 minutes, he will stop near a truck full of explosives. At this point, take out your Welrod (or other weapon if you have no ammo), and shoot the explosives, which will cause the truck to blow up and kill the target. Achievement will unlock here.