Having made your way to the top of the building you will find Enzo. He has a number of men on the rooftop and a helicopter.
If you have a RPG, you can just use that to fire off a round at the helicopter. Be sure to lead the shot as the round takes a few seconds to get there. Alternatively, just blast it with a high power, rapid fire gun. Your target is in the helicopter.
You need to go through all the men who really try to stop you.
Once you have killed your target, grab one of the 2 parachutes on the rooftop. If Franklin doesn't automatically grab the parachute then open your weapon wheel. If there is a parachute icon in the lower right, you have one already. With that, go ahead and jump off. This makes for a very quick getaway.