This has a touch of luck with this but will not be hard.
You must win the duel... If you lose and get a rare card it does not count.
While I said this requires a bit of luck that is a touch exagerated, when you win you will receive at least 10 or so cards and the odds one is a rare card is quite high so realistically you should get this on your 1st win, if you don't just keep playing, you will get it fast.