You unlock the Commando during Operation 2 right before the first stealth mission in that operation. There is an allied unit you get prior to this stealth mission that you can use, but the commando is the cheapest. In order to get a knife kill though you need to be close enough that the commando can attack in the number of movement pieces he has left and you will need to be able to see the knife icon when you scroll over the enemy troop. When you see this icon simply press and watch the fireworks.
Your best bet for nailing this achievement is to use either one or two commandos during each of the stealth missions following the one in Operation 2. 15 kills will fly by. Just remember to use cover, and use that huge distance they have.