Harry Potter and the Half-Blood Prince Tips & Tricks

An easy way to win almost every fight or duel is to cast levicorpus or petrificus-totalus. You just charge your stupify until it chimes twice, and release it. This will result in a one hit victory until later in the game, then it will require two hits.
NOTE: This will not work on Fenrir Greyback.

You can get a large amount of mini-crests when you have used your luck potion and are headed toward Herbology class. At this point, every time you use Diffindo to get the mini-crests to appear, you will get the full amount of mini-crests available, due to your increased luck. At this point you should get as many as possible to help reach the 25,000 piece goal.

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.