This can earned at any time on nearly any mission. Simply shoot an enemy in the legs or body until they fall down. They will either stay seated and fire at you with a pistol, or will roll onto their belly and start crawling away. Despite the description, either position should work just fine. (If one doesn't pop for you, try the other variety) Run up to them and press
(or whatever you have knifing programmed to) to finish them off.
Feel free to ask any questions, and add me on TT/ TA/ PSN/ XBL as Danokitty. Happy hunting!