"A rather psychotic slave trader has been spotted doing his rotten business in the heart of the city. Kill him."
Head to the East of the Pigeon Coop where you get this contract. You will want to head to the Southeastern side of the Restricted area you can find a cart angled against the Southern edge of the building. Use that to get up onto the roof. Go the Northeastern corner and kill the Gunner there. From there just use Eagle Vision to locate your target. You should have little trouble either shooting him or performing an aerial assassination.