Use the following trick if you do not mind getting security after you, or if you are able to do it without being spotted. While he is in the hot tub with his women, shoot into the water. The bottom will fall out like glass with water rushing downward. It drops anyone onto the Pier Floor into the water for an almost instant kill. This includes yourself, so make sure to not be in the water when you do this. This will check off that target as being killed as soon as he dissapears 'down the drain'.