While running at the precise point in the wall press square to do the secondary club attack, then dash and jump in the opposing direction. So in the video I do the club attack while holding up after running into the wall, and then dash and jump down, this technique is known as knockback dashing, it can be performed with the small sword for greater speed, and any other weapon that provides backwards movement when attacking the wall. Basically the attack knockback is added to your dashing speed to sum things up. Also depending on what type of shield you have, you gain increased speed. This trick is incredibly precise. This is pretty much a clip at the bottom of the dragons mouth.