On this, http://pokemondb.net/pokebase/163330/where-can-you-find-a-dusk-stone-in-x-and-y it says that you can obtain a Dusk Stone by defeating the Aegislash balloon in Secret Super Training. I have done that twice (both with a gold medal) but all I get is Stardust. Please help! (For Doublade's sake!)
If you achieve a medium score in the secret super training you get things like stardust, and to get a dusk stone you need to get a top score.