Easiest done in the main bandit camp(though you can get a couple or possibly even all of them in the earlier section in the subway where you encounter them searching the train cars) in combination of the Unbreakable Achievement(disable the truck and get through the camp without taking damage) and remember by "takedowns" they mean sneaking up behind enemies and pushing RS, using crossbow bolts won't count. Also you have to do five takedowns without dying (getting 2 takedowns then dying and then getting 3 takedowns won't count).