He chuckled as he felt her getting weaker, amused and almost high on her reaction. The way she gasped for breath, the way he could see the fear and pain in her eyes.
'You know it takes minutes for a human to die from loss of air,' he whispered huskily in her ear. 'How many depends. Want to find out how long it would take for you? You're a fighter, I bet it would take a long time. How much pain and damage do you think I could do in the meantime?'
He chuckled softly, then moved his wand, whispering a new spell, this one slicing through her robes and skin like a burning hot knife. He didn't make the cuts deep, just deep enough to bleed and cause a maximum amount of pain. All the while he kept an eye on her breathing, ready to let her get some air if she looked like passing out.
He wouldn't want her to miss out on the pain - or to lose her ability to fight back completely. He liked the strong ones. They were more fun. Perhaps he'd even let her have some of that strength back for later. Before he finally killed her.