You can get (or miss it!) at the beginning of Mission "Rock And A Hard Place". To get this Achievement you need to knife (press RB) a Soilder from behind, i think you cant knife im face to face!
Run to the right Side, there will be a scripted Moment, where a russian Soilder knife one of your guys, take him out right after he is finshed.
Hope the Videoguide helps