This one was special
-edit-
For those that didn't get why cutting his hand off worked:
He bound his cut-off hand to his sword, best seen on page 7, which made it look like he was holding the sword higher than he actually was. His working hand was obscured by the bound one, so the demon misjudged Seran's reach. If he just held his sword lower like some here suggested then the demon would've accurately judged the distance.