This one will be a grind, and in all honesty I don't have it yet. When you go on hunting trips, you acquire skill points. Then in the main menu, if you select "Profile" then "Skills", you can upgrade 4 different types of skills. By my count, you will need 124 total skill points to upgrade everything fully. I only had about 36 points when I completed each of the 4 maps on 4 different difficulties, so it will still take a while once you complete all the levels. I would think playing Northern Mexico on Master would be the quickest way to earn skill points, but I can't confirm that. I had the best luck completing levels with the Black Powder rifle, so that would be my gun recommendation when grinding.