This achievement is basically Duane's quest, although it is not officially listed as a quest. When you first go to Duane's dream world, you will see no one. Keep sleeping at different idols until you see Marguerite sitting in a chair with Bertrand standing beside her. Speak to them, and you can request to duel with Bertrand. You have to win duels with him over and over to raise her favor. Each duel you win will raise it by 3%, and each duel you lose will lower it by 1%. You may duel him 5 or 6 times each time you dream, then he will not duel you for around 15 minutes.
Basically, you will get into the duel, and it will either tell you it's Duane's turn to attack, or Duane's turn to defend.
If you are the one attacking, you will see a list of options saying either attack upwards or downwards followed by some kind of taunt. The chance of any of those hitting are all equal, so you might as well just pick the top option every time and just hope for the best. If you succeed, you can attack again, and if you are blocked or parried, you will have to defend the next turn.
For defending, there is a bit more work that is involved, but it's still nothing very tough. Bertrand will say something to you, then it will give you the option to block upwards, block downwards, parry upwards, or parry downwards. Based on what he says to you, you can figure out whether he will attack upwards or downwards. Open the guide found here http://www.gamefaqs.com/console/xbox360/file/932035/51343
(guide created by and credited to CJ Church from GameFAQs), and use the find function on your browser to search for the phrase he says (CTRL+F on Internet Explorer and Firefox). You won't need to type in the entire thing; just type in a few unique words. Be careful though because some of the phrases have a similar phrase to it with only one or two words being different (see the one about the ogre's loincloth). You should get the hang of it after awhile.
After you figure out whether he will attack upwards or downwards, you have to make the decision to block or parry. If you block in the correct direction, you have a 100% chance of success and Bertrand will lose one gem of health. If you parry in the correct direction, you have a certain chance of success (somewhere around 80% for me), and Bertrand will lose anywhere from 2-6 gems of health. I want to set something straight here. I have read all kinds sources saying your chance of a successful parry gets lower as your favor with Marguerite gets higher, but that is just not true. If anything, your chances increase as her favor gets higher. Once the health of Bertrand gets so high after so many duels, it is just not consistent to block him every time; you will not win as often as you would like. I parried every time, and I never lost a single duel. In the event that you do lose a duel, you can dashboard out. The game will auto save every time you exit the conversation with Marguerite, so simply exit the conversation after every duel, then talk to her again to start a new one. That way, it will save after every win, and if you feel like you are going to lose, just go to the dashboard.
You can check your progress by pressing speak to Marguerite. It took me somewhere from 2-4 hours to get the achievement (can't remember exactly how long).