In the first level of the game "Semper Fi", after you are rescued your objective will be "Secure the Village". Once you pass the first couple of huts killing enemies on the sand you will go up a small ramp to get to a hut, stay behind some of your allies. Once on the deck of this hut a flaming enemy soldier will run out of the double door's and attack one of your soldiers. If you kill this burning enemy before he kills your soldier you will unlock this achievement.
This is the link for a video I have created for this achievement.
http://www.metacafe.com/watch/2382323/achievement_guide_cod_...