Once again, set your waypoint for the 'D' on the map and campsite teleport or ride all the way to Escalera to meet up with the Captain. Call your horse and follow the Captain until you reach the convoy in the outback.
Ride shotgun with De Santa and if you can’t be bothered with the lengthy discussion, press B/Circle to skip it. De Santa keeps going on about having a "bad feeling" about the journey, and sure enough, you’ll get ambushed further up the route.
Whip out your rifle and take out all the red dots on your radar as they appear. They’ll appear all around you, so stay on your guard. When you reach the camp up ahead, grab the Sniper Rifle as instructed and use this to pick off the rebels up ahead.
Using the Sniper Rifle can take a bit of time to get used to, as it won’t always start at the same height when you view down the sight (annoyingly). This makes it frustratingly easy to be off target (even if you were looking directly at the target before pulling up your Sniper Rifle).
So try and stay in high cover where possible (such as behind broken doorways) and press up the hill slowly and methodically. Using another weapon tends to prove to be more beneficial if the Sniper Rifle gets too fiddly to use (with the added bonus of saving it’s harder to find ammo).
When you reach the blockade, use a regular rifle to hold back the rebels that break on through and press onwards up the valley, always sticking close to quality cover. If you’re feeling brave, you can sneak up on enemies hiding behind rocks and execute them up close for extra satisfaction.
There’s still a decent fight to get through, so make your way - slowly - through the wasted buildings, up the hill and then towards the fort at the end. As there’s a nice vantage point - and more sniper ammo - in the ledge to your left, now’s a good time to bust out that sniper rifle again.
Once the enemies are all dispatched, storm the fort with your revolver/pistol and use the cover in here to finish off the rebels. Once they’re all taken care of, your mission will end.