Doing this mission the "right way" can be very costly to your bullets. The factory backyard is infested with a flock of enemies and clearing the whole place will most likely be both ammo- and time-consuming.
There is a fast way out of this, which saves your ammo and especially your patience. Arrive at the payphone with a helicopter (The Maverick on top the Vercetti Estate would be the nearest one) and start the mission. Use your helicopter to land on the nearby building on the left side of the factory (assuming that you are flying from the payphone's direction). Snipe every single enemy on the roof where the briefcase is. This allows you to sprint-jump on the factory's roof with no lethal force ahead, picking up the briefcase and escaping to the airport with the helicopter provided nearby.