I missed the instructions during my first duel, but I didn't really worry about it since I just shot a lot at the guy's head, and he died. Many duels went the same way...I could shoot the opponent easily. But during tougher duels, that doesn't work. It seems like you have to have your meter on the right end up higher than your opponents meter...but it's not clear to me exactly how the meter works. And the one bit of advice I remember the game giving me, to "not draw too quickly, because my shots would be inaccurate" or something, doesn't seem to make any sense, since I can manually control where the shots go.
What is the secret to a successful duel?