@morgrim Scenario one or scenario two? Because morals on, Percy wouldn't do that. First of all, the OP didn't say why they are fighting, so we're not sure of Harry's, or Percy's limits. If it's for no reason, Percy sure as heck wouldn't cut off Harry's head.
And like @hardcorefakes and @thedarklordpandamonium said, Gillyweed (Triwizard Tournament, second challenge) and Bubblehead Charm. Also, what Viktor Krum did, Transfigure himself into a sharkhead. However, Harry was not that well at Transfiguration, anyways.