perhaps "The Gauntlet" would be a good name for this event you're describing.

And you do not want to scale the event based on # of players. Bad idea because you want more people participating (instead of guildies saying "dude you're too weak/small, don't join!!")

The game Dawn of Darkness had something sorta similar, they had a Guild (exclusive) boss marathon (like a World boss), once a week. However I like your idea a lot better because it would have many monsters, and more importantly factors in durability drain (so 1 mega-player can only do so much!) as well as recovery time.