Once you go back to the 'W' on the map (this time by the oil fields near Fort Mercer), Nigel will ask you to help him sell a few more tonics to raise his cash flow for the modifications to his cart for the storming of the fort later.
Once the cut-scene is over, ride shotgun with Dickens and get your rifle out to provide cover fire as you make your rapid escape. As long as your targets are within range of your rifle, you can auto-lock onto them and fire at them with confidence that they'll go down for good. Don't forget you can also use your Dead Eye to buy you some time if the enemies get too close for comfort.
Eventually you'll encounter a carriage stacked with explosives in the middle of the road, so shoot it to get rid of it with style and press onwards again. Keep your eyes out for riders in carriages and on horseback until you reach the village of Cueva Seca. Reaching here will signify the end of the mission.