Almost immediately after dropping Seth off, his next mission marker will appear on the map just up ahead of you. So run over to him to start the cut-scene and the last mission for Seth. There’s a few trigger-happy treasure hunters hanging around the derelict town, so watch out for the red dots and use your pistol if you’re close to take them out.
Once the town is clear, press onwards up towards the large house up on the hill and you’ll trigger off a battle with a larger number of well armed hunters. Switch to your longest range rifle and pick them off one by one, using as much cover as you can.
You can run across the open ground (tap A/X to speed yourself up) and take cover behind the stone wall nearby. Use this to pick off the remaining irritations that line up outside the front of the house.
Now run around the back of the mansion, head downstairs and watch out for a couple of goons down here. Give them the good news with your pistol/shotgun and (if you haven’t claimed it already, there’s a treasure under the skull at the back of the basement).
Now run up the stairs and clear the house of any remaining trouble so Seth can open the chest in the final room himself and complete your final mission with Seth.