While in Dumbledore's Office, go to his desk to select challenge mode. Play all of the advanced challenges and beat them for this achievement. It is best to complete the story before attempting these because they are all redoes to minigames during the story. These are all pretty easy and you shouldn't have too many issues. The last one caused me the most issues, I found it best just to spray the spell all over the room in every direction as fast as I could. Using that method I ended up hitting 40 dummies out of the only 30 needed.