When the summoner recovered, he made sure not to use his abilities right at that moment. Instead of that, he just charged normally and then made his body lighter to increase his speed suddenly. At the same time, he made Billy's lower body get heavier and then disappear. It was a simple trick to obtain an advantage and to put Billy at disadvantage as well and it worked more than a little bit.
Billy tried to turn around since he knew the attack would come from there, but the weight on his legs prevented him from doing so. Even though the enemy didn't know how Destructive Aura worked, he already came up with a plan to counter it. It was also pretty annoying that he could make some body parts heavier… Billy always thought that gravity spells would only work in areas of effect.