When the Whomping Willow starts to pound into the ground, hide in the corner where you entered its lair. The shock wave cannot reach you in here, unless you stray too far from it. Should you get hit, you should have refilled your potion Vile before you entered. Also, Hedwig will drop Cauldron Cakes and Beans for your aid. Note: Rapid firing technique is recommended. Lock on to the limbs after they finish pounding and fire a quick shot, then wait to lock onto the Willow's Eye. Then, fire repeatedly until it stops hurting him before running back to your hiding hole.