I found this easier to do on chapter XIII. The chapter where you go to town to rescue the Indian boy. Play as Thomas and as you come to the saloon there'll be people on the roofs shooting. Just kill them until someone throws dynamites at you. At that point Ray will tell you to climb to kill that person, ignore it. The guy throwing dynamites will be on the right side on the roof of the saloon so just stay back so the dynamite doesn't hit you and wait as he throws at you, and then shoot them.