Stock up on weapons and items to have yourself prepared for this challenging mission. When you are ready, talk to the Sergeant in the armory to start out.
There's a lot of zombies along the way, which unfortunately includes the freak types adding more challenge to this task. Follow your way through the warehouses until you get to the door with the opening. Deal with the screamers first to avoid more enemies coming in, then finish the rest to clear the mission.