Meet Micah just outside Van Horn to begin the mission, you’ll need to be accurate with a sniper or like minded rifle if you wish to obtain the Gold Medal here. Ride with Bill to the ambush point in Van Horn and Arthur will agree to be the shooter while Bill creates the distraction.
Head into the nearby house and up the stairs to find the vantage point, getting into cover with your long scoped rifle ready. Target one of the mounted soldiers and wait for Bill to attack first, as soon as he does activate Dead Eye and begin scoring the headshots you need for the Gold Medal. It’s much more useful to stay in Dead Eye and manually pull the trigger rather than painting the targets, if you need to restore your Dead Eye gauge then use any tonics, there’s no penalty for using health items here.
Once the initial wave of soliders are down, jump out the window and head over to the wagon. After Bill drives outside of town, you’ll be pursued by more soldiers as you make your escape. If you have yet to achieve the five headshots then keep your long scoped rifle out and use Dead Eye to go for more headshots. If you’ve already got the five headshots, consider switching to a rifle or shotgun to better deal with your pursuers. You’ll need to be quick, too many shots at the cargo will see the wagon go up in flames and the mission fail. You’ll also need to be accurate so to keep to 85% accuracy.