"We have identified a particularly brutal slave trafficking network operating between Africa and the West indies. The man earning a profit for this villainy is believed to be on a nearby island. Kill him."
Head over to Tortuga. Anchor off the Northern beach to start this contract. Run up the slope to the first Stalking Zone you can. Use the other Stalking Zones to move around to the rear of the house. Your target is back there and is mostly unguarded. Climb up onto the roof after that. From there you can easily pull off an aerial assassination or just dart them.