Upon accepting this contract, a red skull will appear on the map in the centre of Misteriosa in the southeast corner of the world map. A large red, circular restricted area will also now be present in the area. Sail over here and dock.
We now need to enter the restricted area and use Eagle Vision to identify the target. If you have been here previously and used the viewpoint to synchronise the map, you can fast travel to the viewpoint itself, which is pretty much in the direct centre of the restricted area. From here you can easily use your Eagle Vision to identify the target that patrols almost directly beneath the viewpoint.
If not, you will have to come in overland on foot. Either way, you’ll find the target patrolling in and around an area of ruins just to the northwest of the viewpoint. Once identified, climb the ruins to the east near the viewpoint and air assassinate him without being spotted to complete the contract.