Hearthpwn

You are facing 2 problems:

- Majordomos and Ragnaros have a lot of Molten Giant. Are they low on health they can play them and it is hard to deal with them

- Ragnaros has 30 health and 30 shield, so is not easy to kill. Moreover, his hero power is insane. It hits twice for 8 damage.

How to deal with those problems:

- Don't let their life getting to low if you can't deal with the amount of Molten Giant they play. It is important to pay attention, how many Giants they can play. To avoid playing them their Molten Giant, build up a board first, so you can kill them in one turn. That means that you often don't attack the face even if you can.

- Have a full board of minions, so that the probability is low that he hits your face and you have enough minions left to clear the board and do damage. Therefore it is good to have strong minions on the board to kill him soon. How can you ensure to have maintain enough strong on the board? The answer is Kel'Thuzad. What happens if Ragnaros hit him? The answer: Have two of him :)

To sum it up. First gain board control and build up a strong board.
Mulligan: Dancing Swords, Powermace, Lightning Storm and Explosive Sheep are good.
Attack his face only when his health won't be 20 or less afterwards. If you can play Kel'Thuzad you will maintain the board control automatically if your board is strong enough. Before you play him you should make sure that you don't have to many weak minions or minions which summon weak minions on the board, because they occupy the space on the board and you want big minions. Bloodlust is good to clear the board and also to get rid of your totems. After playing Kel'Thuzad you should copy him with Reincarnate or Ancestral Spirit. Therefore it is good to pay attention to the space left on the board. If your board is prepared, you can go for the kill. Either you kill Majordomos in one turn or you let his health go lower, so he can play as many Molten Giant as you can kill and after two or three rounds he runs out of cards.
To deal with Ragnaros is easy now as long as your health is not to low. Therefore play the Antique Healbot if you can (best played before you play Kel'Thuzad, so you can get rid of it, before you prepare your board with big minions). If Ragnaros don't hit both Kel'Thuzad (you can counter it with Ancestral Spirit) or just hit your face you will win the game. 

The deck i use has lots of Deathrattle which has good synergies with Reincarnate and Ancestral Spirit. Especially Sylvanas Windrunner is really good. Therefore i think it makes sense to run Baron Rivendare in the deck.

In the following video i just a slightly different deck, but the strategy and the key cards are the same. I want to apologize for my bad articulation, i was too concentrated on the game :D