His youth seemed to pay off in a fight, something he'd never noticed before. Years of ducking trouble with Fred and George and even more years of ducking his sisters' jokes and teasing gave one a rather good sense of saving one's skin.
He was surprised to land her on her face so effectively, and perhaps it was his slight pause of surprise that kept her Reducto from hitting him full on, just glancing off of him, which in turn caused him to stumble. The stumble got him a nasty slice on the arm, something that bled and hurt but wasn't that serious.
He swore at her and wished he didn't feel a twinge, still, at hexing girls. He sent a stunner at her, followed by a slightly off-aim (reflexes are all well and good, but he was a little rusty) spell that shot ropes to bind her with.