For this achievement, the quickest I found was a Custom Tournament.
Start with 1 player, 1 bot and the combo style set to Fastest Time.
Then press X to go to advanced settings. In the advanced settings, change Lives to 1, Combo limits to Triggers Only, Allow Repeat Buttons to No and Wrong Button Penalty to 0.
Then Select your profile and whichever character you want.
For the bot, make sure the difficulty is set to Very Easy.
From here just start the duel. Once you have finished the first one, just mash A to skip stats and hit rematch. This will load the same settings and a new map.
I was able to do the duels in sub 1 second, with most coming in under 1.2 seconds. The longest part is the loading screens, but overall shouldn't take longer than 30 mins to wrap up all Duel Win achievements.