A "complete" takedown is when you hold X next to an unaware ennemy (on the same level or from a ledge, as long as you hold X you're good).
It'll start a X button smashing "minigame" (?) while Evie is kicking the shit out of her prey.
When the button prompt stops and Evie is high-kicking the poor dude's face, that's 1 takedown out of 30.
There's a challenge asking you to do it 40 times so you'll have an easy way to know what's remaining as it's showing a counter everytime you're doing one.
You'll do quite a few in the main missions and also to work toward the 200 scared but if you're not done by the time you have everything else completed you can just roam around London and beat any ennemy you'll encounter.