After the first duel I had trouble in the next two. I read this on a forum and it helped me out. So if anyone else needs some tips read this
1st: Before the duel starts, hold down the right analogue stick to bring your hand closer to your weapon before you draw your gun. This will help you draw faster.
2nd: Wait until it tells you to draw, before drawing your gun. If you draw too soon your shots will be less accurate so time it well.
3rd. Once it says draw and you press L2 to draw your weapon, a reticule will appear. Use the right analogue stick to aim your shots. Then press the R2 button to place the shot where you want it.
4th. Wait until the reticule is small and white to press the R2 button for more accurate shots.
5th. The two colored bars on the right hand side will begin to fill. As you place your shots on the target the blue bar will fill. As the opponent places shots on you, the red bar will fill. The more accurate and deadly the shot, the more the bar will fill. Hash marks will represent each placed shot accordingly.
6th After you've placed all your shots, the game will determine who wins the duel based on your bars. Hopefully you win.
Winning without killing: Aim for your opponents hand and you can shoot the gun out of their hand. In every duel I've played thus far, the moment I hit the R2 button, my gun went off without letting me fire another shot. So if you do a well placed shot to your opponent's hand, you should be in good shape.
As a side note, the only time this didn't seem to work was in a story duel. I tried over and over to simply shoot the gun out of this character's hand, but did not seem to be able to.
Last, for one shot kills. I haven't tried it since I always go for the hand for one shots, but I would assume one well placed shot to the head would have the same effect. If not just shoot once at their head and wait to see if you win the duel when the time runs out.
I hope that helps anyone who was having trouble.