Rouille parades about the top of the building, nearby the Southeastern tower. He is constantly talking to the decapitated head that he has brought with him.
Head over to the South and get to the stair well. Climb up the nearby wall. This will let you get into a nice position for an air assassination.
You can also sneak up to the platform. From there, get nearby Rouille. Throw a Smoke Bomb. Use that as cover to slip in and quickly take him out. Watch the Cinematic and then it is time to start running.