Description: Kill a Commander with a Stealth Kill
Note: All perk-based achievements can be obtained by re-loading the checkpoint and keeping progress. See video in description for details.
During the beggining of the game you will be prompted about commanders. They're cowardly soldiers that radio-in re-inforcements, and you'll be able to tell when one's about by seeing a radio transmission and a distance count in the top right corner of the HUD.
To sneak behind him, crouch and go up behind him and push the right analogue to activate the Stealth Kill animation. Very easy to do.