This deck has a high win rate against this boss. At first, the deck plays like the typical echo mage deck. The exceptions are, having bigger minions. The reason we want this and not ice lances, taunts, heals, or extra card draw, is because his second form (ragnaros) recovers 30 health and 30 armor.
How to win: You generally play the game by stalling until you can get Emperor Thaurissan. Then you have to stall further until you can load your board with big creatures, with an added ice block. Make sure you can echo Alexstrasza so he can never play those molten giants. DON'T Lower Exectutus or Ragnaros' health below 20, until you have Alex and lethal on the same turn. Rag/Executus does not have enough spell burst so don't worry about dieing with board control.
The early game is the toughest part. Feel free to use Fireballs and Frostbolts on the 3/3s. Spells aren't needed as a finisher. The single ice block can even be replaced if you feel like you do not need it, since if it was ever activated you were probably in a bad position anyway.
A bit about me- I'm a 3x Legend player (2 with priest, 1 with hunter). I like to make outlandish decks. Feel free to check them out. I will also post all my heroic boss decks.