Facing Off
To get the most from the game, it's all about collecting all the available Hogwarts crests. As few as eight crests allow Harry more health while involved in duels, which of course equal more crests when Harry wins. It's best to get Harry dueling as soon as possible to get the crests, thus the health, before attempting the remainder of the game.
The Castle Gates are the dueling arena, so once outside the gates, press Right, Right, Down, Down, Left, Right, Left, Right, Left, Right to instantly unlock the gates. Once Harry is in the arena, cast the "Wingardium Leviosa" levitation spell to smash the shield seal into the adjacent wall, then bring the pieces through the bars to assemble the protective shield.
When Harry is facing off against foes, it's easy to score quick knockouts by casting the Levi Corpus or Petrificus Totalus spells: throw the spell, then charge up your Stupefy for two chimes and let it blast out, leveling your opponent. It works against every adversary but Fenrir Grayback.
Another Method
A non-fighting way to gather crests is to search out boxes of fireworks, hidden throughout the halls of Hogwarts. Upon finding them, get Harry to cast the Incendio spell to set them ablaze. The boxes explode and reveal busted crests. Use the repairing spell to put them back together and add them to Harry's collection.