There's one boss that stands above all others in Final Fantasy XI. Pandemonium Warden. Never seen it myself (never even played FFXI myself), but I'm assured he's a tough cookie. So tough, in fact, that when the guild BeyondTheLimitation tried to take him on the other day, they ran into some problems. Namely, that it was taking hours to defeat just a single form of a monster that had already changed form 20 times. In the end, the guild had to call it quits, because after 18 hours of combat members of the party started throwing up and passing out from exhaustion. 18 hours? We get it Square, tough bosses are tough, but 18+ hours of combat isn't fun. It doesn't sit well with the game's own warning message, either.
How Long is Too Long? [Pet Food Alpha, thanks Brandon!]