Ffter deafeating the Preacher you will win his shotgun.
After the movie sequence, the game tells you to go meet Soapy at the Apache camp. However don't do this right away. Instead stay where you are and walk back up to the Fort.
Two enemies will appear, kill them both and then go back to where the Indians are standing.
Then simply walk back up again and kill the enemies again, and again, and again.