When you enter the chamber after the chess board, there will be a short intermission sequence about Professor Quirrel. After that, he releases about six Mutated Gytrash. In order to defeat them, you have to use Lumos (power up, shoot it, then move towards one or more of the Gytrash). Make sure that you are almost touching them. After you have defeated one of them, Quirrel will try to bring it back. Since you cannot hurt Quirrel while the protection (blue wall) is around him, wait until he puts down the protection. Immediately shoot Flipendo to stop him. Be on guard until he puts the protection up again. Then, resume defeating the Gytrash. Watch out for the Gytrash's "echolocation". If it touches you, it will knock you down and stop whatever spell you were performing. The more Gytrash you defeat, the harder it will be to keep up with Quirrel and the other Gytrash.