Very similar to the "Things Better Left Unseen" or the Rosie O'Donnell achievement as I call it.
The only different between this and my TBLU guide is that you simply must place the body of a dead guard in the other's path while remaining hidden. No need to exert those weak forearms throwing.
You can do this on the first level, again, simply wait for two guards in proximity. If you are worried about light, you can remove the light with a dart, kill and still place the body; the other can see it if it is close in his patrol. If it is not, simply place it near another light to make it easier on the guard (and yourself). Though I believe it worked in total darkness for me.