I found winning at duel to be quite easy and almost guaranteed. Sure u can have bad luck and lose 11 hands in a row(trust me it happened. 6 hands of me having 20 and him having 21), but most of the time u will be on a nice win ammount of glass. I made my purple badge with 20 glass to start and maybe 2 hours time. The trick is to never get another card above 13. If u have 13 or more always show the cards. 12 or below hit. Trust me I even helped a clannie with advice since he was always losing lots of gold on it. The same day I told him how to do it he got purple badge with very little money spent. Try it
PS: I see u have purple badge but u can still net about 6000glass in maybe 2 hours if u wanna break it for dumping achievement