The Eagles Pride guild on Malygos US had an interesting idea for a guild event. They all rolled level 1 Tauren alts and staged a raid boss encounter where they had to take down a level 70 mage. Apparently, a lot of thought went into the encounter, with the mage's gear and abilities tuned to create a realistic opponent for the little cows. Additionally, the mage, Belise, limited himself to using his abilities on certain cues and made good use of emote macros to further mimic a scripted boss encounter.
You can check out the thread on the official WoW forums which includes links to the two videos of the entire event. The thread also contains more information regarding how they balanced the event and speculation concerning how similar fights would work with other classes. It would also be a good place to express your interest if you'd like to participate in another similar event in the future.
I've heard of some interesting guild events in the past, but personally, this one trumps them all. Have you ever taken part in something like this?