To get this you have to perform 50 takedowns which can either be lethal or non lethal. I would reccomend doin mostly non lethal so you can make them go towards the "Consciousness is Over-rated"
"Knock out 100 enemies in a single playthrough." achievement. you must have at least one battery cell charged to perform a takedown and if you have the upgrade where you can take down two guys at once this goes by pretty quick.