Move near a pool of water, and when the salamander turns yellow, move out of it's fireing line and press the A button. Keep firing at it and it will turn black, then lift it into the pool of water with the B button. When you get the water spell, you can just shoot it at the salamander and it will die instantly.
Get cards from fred and george so that you can unlock different spells
When you are in the Prefects Bathroom go on a log and wait for 1 minute. Then start casting spells. If you move around while casting spells Ron and Herminee will rock back and forth and make funny noises.
Get all 99 Tri-Wizard Shields.
Get all 74 collector cards to unlock the movie photo gallery.