This is story related during the sixth campaign mission, Tiergarten Flak Tower and is therefore unmissable.
Proceed through the level, up and over the flak tower, then down it again until you reach the winch room. If you make it across without being seen you will earn this achievement as well:
Next you will see a cut scene and then you need to shoot Müller. He may not be visible initially but if you shoot a few of his guards he will come into view. When making the shot on him remember to press RB for focus mode as it does really help in making a shot of this distance.
Using focus mode also goes towards helping you get: