Hearthpwn

In short this deck wins the fight with multiplying Kel'Thuzad through a Ancestral Spirit and Reincarnate combo as he will not have a way of clearing your board if you have 3 or more alive and from then on the keep multiplying.

The deck was build arround this concept, surviving until you are able to meet this condition through spells that remove Majordomos minions quite effectivly (be aware that some require a +spell dmg totem or minion to make sure they are killed). 

Tactic

Earlygame/Midgame: Focus on clearing his board with your spells, save Lava Burst and Hex for his bigger minions, if you play the next part right you wont need to deal with Molten Giants.

Endgame: Try to setup your win condition if you were able to play Emperor Thaurissan with Kel'Thuzad , Ancestral Spirit and Reincarnate in hand you can do this in 1 turn. If not Play Kel'Thuzad and buff him with Ancestral Spirit and the following turn use Reincarnate on him. Do repeat this a second time and you basically won the game.

Make sure to keep Majordomo at ~20 hp when setting up your board to be filled with Kel'Thuzad so he does not get a chance to spawn molten giants. Once you have a solid board go push for the phase transition, possibly with a Bloodlust and the turn after you should be able to kill Ragnaros.

Mulligan

Try to mulligan for board clears such as Forked Lightning and Lightning Storm

Replacments

Bloodmage Thalnos can be replaced with Kobold Geomancer they are just there to make sure that your Forked Lightning / Lava Shock / Lightning Storm kill the 3/3 minions

Al'Akir the Windlord is also optional but hes a good option for pushing for damage when you Bloodlust or Flametongue Totem