
Maybe it's something that you're not even intended to do, such as killing a supposed-to-be-unkillable enemy. Maybe it's something intended to be beaten by only the most serious players, such as a few challenges they put in the Final Fantasy series (the international version of FFX comes to mind). It could be single-player or multiplayer.
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Here's what I think:
For its time, I'd say the Quarm battle in EverQuest I. I never got to face it, but when the Planes of Power expansion was current, it took months for a guild on Tholuxe Paells to take it out. Leading up to it was a chain of raids, ultimately ending with Quarm, a four-headed dragon (I think they called it a tiamat). Entire raids of 72 maxed-out players were wiped out by it. Today it's not as difficult since the level cap went up a bit, but at the time... Holy fizzlin' BBQ.
