You have to relentlessly try and steal from him. Everytime he speaks and brandishes a new sword that means a new item can be stolen. It's tough and doesnt seem to have the highest of probilities because I only got 3/4 genji items even though I used thiefs cuffs and had Vaan only ever trying to steal from him for the whole duration of the fights lol.

Well done anyway, certainly isnt easy at that level, I was 60+ when I beat him. Good luck with the other marks, which are harder