Get inside the factory from the rooftop or from the Western side of the factory. Both entrances have a Watcher guarding the entrance. Move in carefully and assassinate either of them then get inside the factory.
Inside, use Eagle Vision to identify your target. Now you have a few choices. You can clear out the various Blighters from the area quietly using Whistling and assassination or you can clear who you need to and sneak up to the Factory Foreman.
Pick off the two Blighters on the same floor by luring them over to you using whistling. Try to get each in the center of your screen as you whistle to aim it more at them.
Done properly you can easily slip inside the office with the Foreman, kill him and steal the deed.