The best part of the game if you just want to rinse and repeat this is in the Monaco casino gardens just before the third and final security camera. There is a checkpoint then you walk down a set of steps and round a corner to your right, two guards are talking, on the far side of a low wall. Wait for one of them to step forward, then do a stealth takedown over the wall on the one closest to you. Immediately hide behind the wall. The second guard will turn around to look for his friend who seems to have disappeared, and walks close enough to the wall that you can grab him too for a second reach over cover takedown. Then just reload the last checkpoint, which is immediately before this action as often as you need to.