A Saint's Battle with the Prince of Darkness

I first heard this mysterious saint story when I was sitting around a campfire with a group of friends at a Catholic Call to Chivalry Camp. No one could remember if it was Saint John Vianney or Saint John Bosco, but I’m going to follow it up with a fascinating true story from Don Bosco’s life which I think corresponds to it. If you know for sure which saint it was, please help me out in the comments.