Go behind a pillar, and when it rocks, cast a Flipendo spell on it. If timed correctly, it will fall on him. Keep doing this and when all the pillars have been used, use the Flipendo spell to direct it into his green spell and it will backfire. Go behind the broken side of the mirror and when he tries to hit you, it will sometimes hit the cracks and backfires.
When you are chasing Malfoy in Remembrel Chase, speed up your broom stick. Try to keep Harry ahead of Malfoy so that Malfoy chases Harry, When he tries to go ahead, collide with him. Do this repeatedly until Malfoy loses all his health. You can do this trick when you are chasing Key and playing a Quidditch Match.
During broomstick training fly directly at the Hogwarts seal. You will enter a secret room and find four Bertie Botts every flavor beans and a Wizard Card. The beans will be quite old seeing no one has found the room since 1867.
After learning the Alohamora spell, when you enter the room with Hermione for the test, cast the spell on the statue to get a bean and open a secret room with a magic card.
When you first enter the garden after defeating Malfoy, finish talking with Hagrid the look for a fountain. It will be on the right side of you. Cast a spell on it to open a secret room behind it. Collect everything in the room, then cast the spell again to close the room.