I am going to say to you, the quest designers, the same thing I would say to any railroading Dungeon Master: I'm here to play a game, not read your novel.
Encounter the Medusa - cannot hurt her
Encounter the Medusa - cannot hurt her
Encounter the Medusa - cannot hurt her
This is bad quest design, and it goes all the way back to Information is Key and the bandit who closes the gates. I can move faster than that guy, but I cannot hurt him.
If you want to tell a story that goes the way you want it to, there are plenty of self-publishing options available these days. Go, write your story. But please, please stop making poor decisions like this. If you don' want us to kill the Medusa before the time you appoint, THEN DON'T HAVE HER SHOW UP UNTIL THEN. Magic mouth is a wonderful spell.
No, for real. This is just lazy design.