The tutorial in the game isn't the most detailed and some might find it hard so I've written a mini-guide on what I've learned so far.
To win a duel all you need to do is fill up your bar (blue bar) before your enemy's bar (red bar). To fill up your bar quickly you need to mark as many shots as you can and as accurately as you can.
1. Drawing - At the start of the duel you and your opponent will draw your guns. You're actually at a disadvantage if you draw first as you get an accuracy penalty. Wait until your opponent goes for his gun and then draw.
As for drawing controls, I hold L2 and then mark with Right Stick and R2 but this is just my personal preference.
2. Marking - In this part you mark your shots on your enemy. To make your shots accurate you will need to time your shot when the crosshairs are small and white to get the best accuracy and boost the bar.
I find that making a couple of accurate marks is better than spamming inaccurately as you can get lucky but you won't always win as tougher enemies boost their bar faster.
I've only lost one duel and it was for a Stranger mission. I tried to just disarm him, but I died several times. I had to lethally paint him for me to win the duel. Unless the story moves in two different paths, then that's just the way the mission went.
I've won several duels by painting one or two targets and waiting it out... I really don't know how I won those I guess. --- My fat kid tried to eat the honor roll.
On the two meters, a red and blue one, when a person marks a shot it makes a "tick" according to the time. Then when the timer goes off the "ticks" are fired off in order. So make sure your first shot (tick) is either immediately lethal or a disarm, or else you may be killed if your meter wasn't full. --- My fat kid tried to eat the honor roll.
Usually aiming straight for the head and hitting as fast as possible will win you any duel, but if you quickly move from draw to their gun hand, you can disarm them in a single instant shot. Lets you be honorable and still win.
--- GamerTag: Te1vanni Guard "Telvanni you would disprove everyone. :D"- Saint_of_Misery