"This treacherous pirate captain wishes to capture fort Chinchorro for himself. Don't let him."
You must have conquered the Chinchorro Naval Fort to complete this contract.
Fast travel to this area. Climb up to the Fort Commander's office, on the Northern side of the Restricted Area, and deal with the Gunner there. If you listen then you can hear the approach of your target. Identify your target. You have a range of options open to you: pistol, berserk dart or Air Assassination. Choose your favorite for what he has to say and dispatch him that way.