Unlockables

Collect the indicated number of Hogwarts Crest to unlock the corresponding bonus:
Dungbombs in prank boxes around Hogwarts: 21 Hogwarts Crests
Exploding Cauldrons in prank boxes around Hogwarts: 48 Hogwarts Crests
Love Potion in Potions Club: 98 Hogwarts Crests
More health in duels: 8 Hogwarts Crests
Even more health in duels: 71 Hogwarts Crests
More mini-crests with each cast: 129 Hogwarts Crests
Score boost in flying events: 38 Hogwarts Crests
Two Player Dueling Pack 1: Draco and Luna: 3 Hogwarts Crests
Two Player Dueling Pack 2: Crabbe and Goyle: 29 Hogwarts Crests
Two Player Dueling Pack 2: Training Ground Dueling Arena: 14 Hogwarts Crests
Two Player Dueling Pack 4: Paved Courtyard Dueling Arena: 113 Hogwarts Crests
Two Player Dueling Pack 4: The Transfiguration Dueling Arena: 59 Hogwarts Crests
Two Player Dueling Pack 5: Ginny and Hermione: 84 Hogwarts Crests

Knocking down Crests

Locate the Crest you want to knock down. Cast Wingardium Leviosa on the object you want to use to knock it down. Note: This object can be anywhere and does not have to be in throwing range of the Crest. It is recommended you use either a Dung-bomb or an Exploding Cauldron. Once the object is in the air, hold [Run]. You can now walk around the castle and grounds while still carrying the object in the air. During this time Harry will not run fast. Make sure you keep [Run] held until you get to where the Crest is located. Throw that object at the Crest to knock it down.
Easy Mini-Crests

You can get a large amount of Mini-Crests 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 will get the full amount of Mini-Crests available, due to your luck. Get as many as possible at this point to help reach the 25,000 pieces goal.
Easy fights and duels

An easy way to win almost any fight or duel is to cast Levi Corpus or Petrificus Totalus. Then, just charge your Stupefy until it chimes twice, and release it. It will result in a one hit victory until later in the game, then it will require two. Note: This does not work on Fenrir Greyback.

Use the following combo to defeat any enemy. Use Expelliarmus to knock them down. Charge up your Stupefy. While getting it charged, walk directly next to your dueling opponent. Fire your charged Stupefy for a one hit kill. This trick can give you five stars for the challenge very quickly and easily.