I did manage to beat the Incubus on B6 and got the Incubus Wings. Because the Incubus is woven to the story I don't think I will ever see another one and therefore get no other wings. I also can't sell it to the material shop to get more. So, it's only a single use and I want to make the best out of it. Is a magic stone the best I can do with it?
Making a magic stone is all you can do with the Incubus Wings. Additionally, there is no magic stone that can only be made with the Incubus Wings, so there will always be other options available to make any\all of the magic stones in the game.