The description for this achievement was confusing at first, but just to clear things up, it wants you to first snipe Kreidl, then leave the building and kill all of the enemies from the convoy.
This achievement can be unlocked on Mission 1: Schoneberg Convoy. After you have set the charges, shot the explosives to stop the convoy. When Kreidl comes out of the truck, ONLY SHOOT HIM. Do not shoot any other enemies while you're still in the building. After you have taken him out, run down the stairs and out the building. Now take out the rest of the enemies from the convoy and the achievement will unlock. There will be a small tank cutscene, but you will be able to keep killing the enemies right after.