As of this writing, Method is the only group to have defeated Archimonde. The group is keeping its, well, methods under-wraps for now, at least as far as the final boss, Archimonde, is concerned. Blizzard Watch notes that Archimonde is the first raid boss to survive raid group onslaughts for weeks following release since Throne of the Four Winds’ Al’Akir, back in 2011.
More information on the Hellfire Citadel’s boss fights will certainly pour out over the coming days and weeks. As WoWWiki notes, it is not yet confirmed whether Archimonde is dead for good or not. That’s of little concern for raiding players right now, however, particularly those in the US, for whom the first-to-complete slot still looms open.