Even though the hardest difficulty for Warlords of Draenor's current top raid, Hellfire Citadel, has been available for a month now, only four guilds could kill all 13 bosses inside. It's an extremely hard fight, but things might speed up as the two top guilds, Method and Paragon, have released their kill videos.
As you can see below it's still just Method, Paragon, a Russian guild and a French guild who managed to complete the raid on Mythic. Although it's a well tuned fight, it's not just about having superb gear, but also excellent coordination and having the ability to focus for more than 10 minutes at a time.
They killed Archimonde two weeks ago, and the usual way is to wait with releasing kill videos till at least five top guilds defeat the boss (so they don't spoil the strategy), but since other guilds started to stream their attempts and Paragon's already streaming full clears, they didn't wait any longer.
Here's Method's kill video and man, that last, Mythic-only phase looks incredibly hard. You have to watch out for massive orbs, knockbacks, summoned infernals, huge amounts of debuffs, and in the meantime you still have to do crazy damage to the boss. No wonder it took 472 tries for Method, 675 for Paragon and 749(!) for the Russians to make it through. Just for comparison, Paragon wiped only three times during the first three bosses of this raid, so yeah, this boss is tough.
Shortly after Method, Paragon, who came in second, released their kill video. Their strategy looks a bit more organised (maybe it's just the addons) compared to Method:
And here's a bonus: French guild From Scratch also defeated Archimonde. No video yet but they have... nerdscreams: