This can be done at towards the beginning of the mission "Operation Tonga", when you get to the farmhouse and you first approach the fence and wait for the small convoy to drive by you will go towards the right by the wagon. Watch the 2 guys inspecting the body on your left, the one on the right will start looking at you as soon as you get to the wagon. Sit behind for about 2-5 seconds and then move outward towards the right and just before you get to the fence start to crouch and wait about 2-6 seconds depending how fast you get there or wait till the German on the other side of the wagon starts to turn around and walk. There is a German inspecting stuff inside the back of the wagon, jump the fence and do a takedown on him. After you assassinate him you can immediately loot his body and then pause and restart last checkpoint which will spawn you back at the fence. Now after getting 60% of this achievement done this method it didn't let me loot his body so then if he doesn't drop anything then go after the German on the other side of the wagon that starts walking towards the fence line and do a takedown on him and he will be able to be looted. Then all you from there is rinse and repeat and there ya go.