UPDATE: I challenged level 30 Kaiba many MANY times and NOT ONCE did I get Flute of Summoning Dragon or any other rare or super rare cards.
HOWEVER, after dueling kaiba at lv 40 for the same amount of times I managed to get TWO "Wingbeat of Giant Dragon" and TEN "Flute of Summoning Dragon"
Red Eyes B.
You can also win it by vs. other players in ranked duel matches.
Basically you win a prize that looks like a ticket. You can then exchange that ticket with a cad of your choosing depending on the CLASS of the ticket (R, SR, etc.)
You can exchange the ticket at the page where the card trader is. There's a small black bar that appears just to the upper right of the card trader icon that lets you select it in order to exchange the ticket.
The bar only appears when you are in possession of the ticket.
20th WIN = ticket(R) -> Can be used to claim Flute of Summoning Dragon.
40th WIN = ticket(R) -> Can be used to claim a second Flute of Summoning Dragon.
After that you'll have to wait till' things get reset in coming October before you can try again for the third ticket(R)
you have to get it straight from mining Kaiba.