Simply load up the mission "Winter Contingency", this has to be done on 'Legendary' difficulty. Sprint forward as soon as you gain control of your character, down the hill to see a camouflaged elite. You need to kill him before he gets away.
Once you have killed him, you may also proceed to view the data pad to unlock another achievement:
As you can see if you fail to kill him you can simply reload the last checkpoint to try again. If you get a bad checkpoint where he is too far away you can also restart mission.
I created this video to help: