The Trouble with Sci-fi/Fantasy Villains

I am hosting a discussion on "Defeating the Undefeatable Villain" over at the Enchanted Inkpot if you want to stop by to see what some of my fellow writers have to say about this issue. You know, when Everykid must save the world from a terrifyingly powerful foe with a name like, say, Dr. Evil or Voldemort... It's not easy to write a book where this scenario feels credible, so how do writers handle it?