Someone correct me if I'm wrong, but you cannot get this in the Guam mission. I think I got five KO takedowns after pressing the Attract button and nothing popped, but first try in the Benghazi mission and I got it.
Anyway, as soon as you see your first militia member in the Benghazi mission, get in cover and press Select. The target should look around and slowly approach you. As soon as the screen prompts you to press X for a takedown, do it and the achievement should pop shortly after.
Helpful tip I discovered while playing through the mission - if an enemy doesn't respond to your first sound don't keep pressing Select - once too often and every enemy in the area will converge on your spot, no matter your visibility.