Did anyone mention Demogorgon from Nethack? I'd say you stand about a 0% chance of surviving a fight against him unless you read strategy spoilers. Even then, if you aren't prepared to take him (he generally shows up randomly), you're probably fucked.
There's an old spoiler FAQ about him here:
http://www.steelypips.org/nethack/demogorgon_faq.html which probably won't mean much to you unless you know Nethack, but rest assured he's brutal, especially considering if you die, you're dead and have to start from the beginning of the game again.