I'll try to do this as spoiler free as possible.
This is the next mission after you get the "sons of Mexico" achievement. It is story related so it cannot be missed. The mission begins in the westernmost town in Mexico, the mission is very straight forward. The mission begins with a duel, (Aim for the head and try to draw first, I had to try this several times before I killed him). Shortly afterwards you must climb a hill with a gattling gun firing at you along with other enemies, I recommend sticking to the wall on the right because the gattling gun won't be able to hit you from there. The next part that will give anyone trouble will involve chasing a stagecoach on horseback, (the stagecoach has a gattling gun on the back) just use dead eye to kill the guy in the gattling gun, otherwise he will probably kill you or your horse.
At the end of the mission there will be two men for you to kill, though you only are tasked with killing the one on the ground, I recommend using dead eye and head shotting them both. When I simply shot the man on the ground the first time my game glitched and the cut scene was not triggered, Marston ended up standing there for a minute, unable to move, before I had to restart the game.