For this takeover, I’d recommend starting from the viewpoint just south of the restricted area. From here, move up to the roof of the building directly in front and kill the sentry here. Turn to the left and take out the sniper on the adjoining rooftop ion the distance. Return to the first roof and use the rope to access the central building with the flag. Interact with this to complete the first objective.
From the rooftop with the flag, look down to the ground below on the west side. You should be able to spot the barrel we need to destroy right away. Hold here for a second however and activate eagle vision as the gang leader will patrol by the barrel quite regularly. Once you have spotted and tagged the leader, shoot the barrel to dispose of the guards nearby before dropping down to kill the main target. Alternatively, you could wait for the leader to patrol of, air assassinate him and then run around and shoot the barrel – either way you choose to do it is fine!
Once all three objectives are done, the takeover will be complete.