"A Spanish soldier guild of war crimes against innocents has deserted and is believed to be hiding nearby."
The contract itself is completed on Salt Lagoon. Head to the rock "archway" that is found in the Northern part of the area. Your target is alone in this area. You can move up easily using the Stalking Zones and take him down without being detected. Otherwise the best thing to do is just chase after him and run him through. He is not very fast making for a short chase.