There is actually a much better and faster way to get a Dusk Stone. All you need is a Pokemon with its EVs maxed out. Go to super training on your PSS. On super training, make sure your Pokemon is in front. Do super secret training until you get to the aegislash. Try to beat it as fast as you can. If you beat it fast enough, you get a dusk stone.
Hope I helped!
Source: Experience
