In a situation like that, outnumbered 50 to 1, if the PCs decided to fight, I'd have them grappled to Hell and hung from the nearest tree without a single dice roll. A little House Rule of mine states that if the situation is too stupid to be believed (eg., a level 20 Fighter cutting down 2billion peasants with Great Cleave in one round), the LOGICAL outcome will be enacted instead. I give very specific examples of this, including Monks getting grappled to Hell by 5-6 guardsmen REGARDLESS of the level of the Monk. I don't care how good you are, but when you are buried by bodies, you are BURIED.