Defeating The Whomping Willow:

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.
When battling the Whomping Willow, Flippendo his hands when he punches them down, then run to where Ron is located and Flippend near him. He will lose some of his life bar (bottom right). Keep repeating this until he starts throwing rocks at you. Dodge them by running left. Then, Flippendo his hand very quickly and Flippendo the location near Ron. He should be defeated after that.
When fighting the Whomping Willow, strike either hand with the Flipendo spell when you see the light on his hand. He will start moving his hands around. When he does. strike the eye in the center of the tree. Keep repeating this process and you will defeat the Willow. Watch the bar in the top right corner that will tell you the Willow's SP.