I recently got a Shiny Litwick, and I decided that it would look better as a Chandelure. I needed a Dusk Stone, so I went over to Secret Super Training and fought Aeigislah around 7 times. Each time though, I would get Star Dust or a Hard Stone. I checked the Wiki, and it says that the Dusk Stone is a high tier score. I always get it in over 2:40 Min remaining, what decides that I get a Dusk Stone instead of a lower tier reward?
There are a couple different ways to get a dusk stone.
You can get one from the Team Flare member in Laverre City AFTER you beat the Elite Four. He will be standing right on the path right before the Pokeball Factory.
Defeat the Psychic Inver guy on Route 18.
Find the stone in Terminus Cave
Beat the Aegislah super training with a time of 2:10 - 2:20 remaining - It's not a guarantee to get the stone even if you beat it within that time. But it is the requirement to be able to get it. Just like other super training games you must beat within that time to get other stones.