Whilst this headquarters takeover is classified by the game as medium, I personally found it to be the most difficult of all of them. This is because the gang guards here seemed particularly clued in and were able to spot me randomly whilst I was in stalking zones or on rooftops out of sight despite my best efforts to stay hidden.
To complete this one, I recommend forgetting about the stealth and instead performing a couple of hit and run attacks. To begin with head in go to the very northeast corner of the zone, climb the building here and kill the scout on the roof. Enter the restricted area and follow the boundary to the east. Climb the large building here and dispatch the sniper on the roof.
Make your way to the far left corner and look down below. By now the gang leader probably will have been alerted to your presence; however he still has that green search area surrounding him so that you know where his general location is. Switch on your eagle vision and form the corner here you should be able to spot him hiding in the shrubs by the fence below. Air assassinate him.
The final two scouts can be found either side of the large building that dominates the eastern side of the restricted zone. Run up and kill both of them. Finally, head to the very centre of the area, climb to the flag, kill the sniper standing on the platform next to it and then cut it down to end the takeover.