I'm sure there's no level requirement for getting a duel as a drop, as my storm got a master duel fighting jade oni when she was... lvl 38 I think, and an archmage duel fighting meowiarty at lvl 28 or 29.
Oh sorry, I read that as minimum instead of maximum. I don't think there's a maximum level, though I'm not 100% sure on that, sorry! If there is one it's above lvl 90, cause my highest level wizard was farming jade oni during Christmas in July, and got a master duel out of it
There isn't a level you have to be to get these duels, or a level where you're too high to get those duels. However, I have noticed that higher-leveled wizards are less likely to get the Master version of those duels and more likely to get the Exalted version.
It's not a level based drop, you can get rattlebones mastery simply from defeating rattlebones in Unicorn Way. I urge you to be at least in the mastery level before going into the dungeon, though.
