Harry Potter and the Half-Blood Prince Hints

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

When you see the shields behind the locked gates, use Wingardium Leviosa and hit it into a wall to break it. Lift the pieces out through the gate and repair the pieces to get a shield.