Beat The Whomping Willow:

When the Whomping Willow pounds the ground, hide in the corner where you entered the lair. When it stops pounding, lock on to the limbs and hit them quickly. Then lock onto the Willow's eye and blast it until your attacks stop damaging it. Watch for Hedwig dropping Cauldron Cakes and Beans to help you out. That should help you beat the Whomping Willow.