Get full armour and health before traveling to the port. A minigun will be useful. Once you reach the docks, either shoot the gunmen or run them over with cars.
Follow the dot into the shed where 3 hammer-wielding mechanics will try to kill you. Dispatch them and activate the switch. Once you do, a few more gunmen appear, but you can usually survive the run straight to the boat.
You'll get a three star wanted level. Drive the boat straight to the mansion to win the mission.