This is a video link to a series by Seraphim17
(recorded on the 360) of him getting this achievement without much of anything. Really easy to follow and with great commentary.
One important note about the last video of the set. If you are playing on easy, as I was, the captured assassins never seemed to be killed and therefore you couldnt fail, and therefore you had to rescue each one. Follow the path as per the video but make sure you rescue each one and dont worry about the extra music men.