Here is a full walkthrough with all achievements guide in Gun Done.
The minigame against him is simple: when the clock turns, shoot.
To get the 0.01 sec mark, you have to anticipate the opponent’s shot well. There are two good duelists options for this: the hooded guy and El Diablo, the guy with sombrero. Against hooded one, use the flag item for him to see where the stick will be throw and you have to shoot exactly to this point. Against El Diablo, use the glasses item for him to see how many steps he will make and try to anticipate after the zero step count.
The duels tend to get harder as you win consecutively, but win 18 is reasonably easy after you learn the minigames’ concepts for each and have gold to buy items that will facilitate your duel.
Death as duelist will appear randomly as the others. It’s very easy to defeat her. She will walk to you fading out for some seconds and when she materialize with her colors, it’s your time to shoot.
In the main menu, go to Heroes, select Gabe as duelist and win 222 duels with him.
There are heroes that can be bought and others that are unlocked after fulfilling certain requirement. You want those second. They are the duelists from the singleplay where you need to win some duels against them and others from more general requirements like winning 900 duels in total.