You can get MASSIVE amounts of Mini-Crests, however you can only get it the one time only, and that's only when you have used the Luck Potion and are making your way to Herbology. At this point, every time you use Diffindo to get the Mini-Crests to appear, you'll get the full amount of Minicrests available (due to your luck). You should at this point get as much of it as possible, to make the 25,000 pieces that you need an easier target to reach.
You can QUICKLY and easy dispatch ALL enemies by following this three step combo:
- Use Expelliarmus to knock them down
- Charge up your Stupefy, whilst getting that charged, walk RIGHT NEXT TO the person you're duelling against
- Fire your charged up Stupefy for a OHK
This has worked each and every time, and can give you 5 stars for the challenge VERY VERY QUICKLY & EASILY