This could be like Strongest Disciple Kenichi’s game of rock paper scissors: if hero boy aims for the moment ripper strikes, then ripper just has to fake that strike, but hero boy can try to also feint slightly delaying the strike and forcing ripper to not strike then continue slightly further, then the hero looks to bait ripper into a moment to strike, which ripper would have to read and make a fake strike etc. etc.
Or it could be that hero boy just ain’t fast enough lol.