I got the El Patron Sword off of Darkhart myself, it takes alot of patience and a little bit of luck to get the good stuff.
Try following my theory, it's gotten my 3 good weapons so far:
Use pistol, lead shot, doesn't matter what kind - repeater I prefer.
Do not use blunderbuss, the shorter in time you kill him the lower the rating of the look goes (for me anyway).
The longer it takes for you to take him down, the more likely you are to get better loot.
Also most people try this theory and just give up, you don't wanna do that. Keep trying and trying it. After all, it's worked for me.