Hearthpwn

I built this deck off of the core of my heroic Baron Geddon deck:  http://www.hearthpwn.com/decks/226711-heroic-geddon-fatigue-druid

One-shot Heroic Ragnaros with this guy.  The strategy is to maintain board control while milling Rag down and building a solid base of giants on the board to knock Majordomo down from 20 health to zero in a single turn, and then kill Ragnaros as quickly as possible thereafter.  You will spend many turns with a board full of minions and Majordomo at 20-30 health just biding your time until you draw the savage roars you need.

Sea Giant and Clockwork Giant are your main workhorses here.  Use him to kill Majordomo's minions and then return him to your hand with the brewmasters and re-drop for cheap.  The plan isn't to kill either boss with fatigue damage, so much as it is to fill their hand and make them burn cards while simultaneously filling your hand with more giants to drop.  Majordomo, especially, refuses to play cards early, so forcing him to burn a few with a full hand is great.  Use Naturalize to maintain board control.  A full board is greatly in your favor here, even when you're just biding your time until you have both Savage Roars in your hand.

TBH I never used Tree of Life in my Ragnaros kill, but it seemed worthwhile at the time.  Feel free to swap it out with 2 Mountain Giants or other big bodies.

Once you get Majordomo down to 20 ish health, have both Savage Roars in your hand, and have a full or nearly full board, go for the face!  You should easily be able to top 30 damage in a single turn, which is enough to put Domo down and drop Ragnaros to at least 50 health total.  Follow up the next turn with another Savage Roar blast and he'll be down to 20 health, maximum.  Regardless of which minions he nukes in those two turns, you should be able to blow through him with ease on the next turn.

Note that it doesn't matter how low you drop Ragnaros with your second Savage Roar as long as you have the board presence to finish him off the next round, regardless of which minions he nukes.  He doesn't have any taunters, so even if he dropped 8 Molten Giants, it wouldn't save him.

Good luck!