The startlingly NSFW webcomic Oglaf had a similar situation once, involving a monster that was real, but appeared fake, like one of those Chinese festival dragons operated by multiple people. It couldn't be killed by ordinary means, but someone figured out that it took a "fake" weapon to kill a "fake" monster. (I'm not explaining it well, but it makes sense if you read the comic.) I'm presuming Luz' plastic sword succeeded on the same principle.