I saw this tip at another site, and can confirm that it works.
While dressed in a Lady outfit, go to the Assassin HQ (marked by a white Assassin icon on the map of New Orleans). From the map, put the blue circle marker on it to make it easier to find. Then, in the vicinity of the HQ, look for red dots on the map, marking where guards are stationed. Go cause a ruckus and get them to chase you back to the HQ. Run up the ramp that leads around the side. At the top, go right, until you're at the end where a rail blocks the way. Now, just kill the guards. It only takes seven to complete the trophy.
What didn't work for me was trying to kill guards while I was on the ground (ex: shooting them with poison). I don't know if this means poison doesn't count, or if it simply means both you *and* the guard have to be "on a rooftop." But this solution works.