Expert Sniper Achievement
Eliminate MüllerThis is story related and cannot be missed. This can be earned on any difficulty.
Towards the end of Mission 6, 'Tiergarten Flak Tower', after finding the winch room in the flak tower, walk over to the objective marker and a short cutscene will play. Colonel Muller is located at the top of his command tower a good distance from the building you're in. He is identified by wearing a black officer's coat and no hat.
Since there won't be an objective marker on him, use your binoculars to locate him. If you're playing on a lower difficulty, tag him to mark his position then shoot him and the achievement will unlock.
Note: At the distance you need to shoot him, there might be considerable bullet drop, even on lower difficulties. Adjust your aim appropriately if needed.