Head up to the ledge above you. Run across to the building with the open window and head inside. Run straight through and out the next window (or just run over the rooftop). Take out the guard that is patrolling on the rooftop. This can be done with a simple assassination or a quick shot with the Phantom Blade. Continue forward and straight up the next rooftop. just keep going until you get to the very top of the building.
At the very top of the building you will find an open hatch on the roof. Head over to the right of that and look down. You will see something of a ladder and an open window below it. Slip inside and take out the Extremist who is over to the right quickly. Start sneaking over to the nearby stairs. Take them down. At the bottom there head over to the right and then go down the corridor to the left. Go to the end of the hallway and over to the closed door. This has your target inside it.