After you finish your phone call, get into the car with Dennis. When you reach your destination, go up the road. Change sides to the right and hide in plants. Approach the outpost as close as you can and use your camera, in order to mark the positions of as many enemies as you can.
As you follow the fence to the right, you will find a hole. Right after you cross it, quietly kill the pirate, who should be cleaning his weapon here. Use the camera again and look around the area. Make sure that the area is not patrolled by a dog. To your left, there should be yet another enemy that can be taken out without alarming the remaining guards.
Wait now for the rest of the enemies to turn their backs at each other and try to take them out quietly, too. If you cause an alarm, Dennis will help you fighting, but you will have to mind his health bar at the top of the screen and make sure he doesn't die. Disposing of all of the pirates, is equal to securing the outpost, Enter the marked building to complete the mission.