Harry Alonzo Longabaugh (1867 – 1910), better known as the Sundance Kid, was an outlaw and a member of Butch Cassidy's Wild Bunch, in the American Old West.
Call of Juarez: Gunslinger
The Sundance Kid is mentioned several times throughout the game and takes part in the stages "Not so Great Train Robbery" and "The Good, the Bad and the Dead." He is both a traditional boss in the sense that he has a health bar and must be fought like other enemies, and also is the level's ending duel boss, along with Butch Cassidy.
The Sundance Kid will always spawn in the gravestone furthest away from you, allowing you to dynamite where he comes from and get some early damage. There is limited cover, so only the gravestones really supply much of anything useful (going further away will get a pop up "You're leaving the story" before the game will send you back to the checkpoint before the boss starts). Also, ammo is somewhat limited, though you should still have plenty from the piles of it found in the middle of the area.
Peek in and out of cover, rifling him down or using whatever weapons you've brought along while he approaches. When he gets too close, dash to new cover, not spending too long out in the open. Especially on higher difficulties, this can easily get him to trigger Sense of Death, or kill you if it has already been activated earlier. Concentration is important, since you can't gain any more from this point on (you only gain it from kills, not damage), so try to conserve it before the fight and not overuse it during the second half of the ghost section.