Once you have found the caravan, the Skulls will appear when you approach it. They are very tough and can take multiple hits with a rocket launcher; and you can blow up the truck you should retrieve while fighting.
So it is best to not fight them at all.
Make sure you bring your horse to the mission
Your best bet is to first approach the two tanks in the caravan and extract them using the fulton system. If you crawl, the enemies will not be alerted.
Then you approach the truck, preferably when the Skulls are not standing around it and use the fulton recovery on it. The Skulls will now be alerted, so you start running in any direction, while calling your horse. Hop on it and ride as fast as you can.
The Skulls will keep up with you and shoot, but most shots will miss. When they are in front of you steer to another direction; just try to keep in mind you want to reach the border of the mission area, so it is wise to keep roughly the same direction overall and not ride in circles. You will no have time to consult your map, so just ride as fast as you can and avoid obstacles.
Once you crossed the border of the mission area, a short cutscene will follow with the fog lifting and the skulls disappearing. Mission complete.