When you came face to face with Prof. Quirrel, he releases some gytrashes and realeases a BlueRing around him and the Mirror of Erised. To get rid of the Gytrashes, do not first collect one Chocolate Frog first. Charge up with Lumos spell and point to Gytrashes. If half of their life was taken, Quirell will turn off the shield and he's casting a spell to add some more Gytrashes. While he's casting the spell, cast him with Charged Flipendo. Do that many times until you finish that part.