The impact of her magic did send him back a little, but the demon didn't even lose his footing. He merely slid a few feet, arms raised out in front of him. Head lowered momentarily, he slowly lifted it upward to look at Willow, the corner of his mouth tilting upward into a vicious sort of grin. He then slammed a foot onto the ground, causing the earth around them to literally shake. The house swayed violently and the ground beneath Willow suddenly sent an invisible sort of wave upward to push her toward -- and hopefully through -- the porch roof.
He knew that he'd have to tire her out a little before the spell kicked in. But he was perfectly okay with that. He'd rather have it this way. A good fight and then an even better fall. "Come on, witch!" He bellowed, leaping from the porch and onto the lawn. "Do your worst!"