So Chromaggus gives you these horrible 3-mana spells every turn. Total nightmare to deal with.
The only win condition I found for this boss (apart from Buff Priest...again...) was Mana Wyrm plus Sorcerer's Apprentice plus a lot of praying. The problem is, those cards will die eventually. When that happens, you quickly find yourself up the proverbial creek without a paddle.
Until Summoning Stone came along. All of a sudden your late game is all about churning out endless bonus minions that can easily deal with your opponents. Disposing of Chromaggus' spells will give you a 3-mana minion every time, making board control a breeze. The Summoning Stone itself will even survive Flamestrike or Swipe, giving it enough staying power to take you to the win.
This is the most fun I've had in Blackrock Mountain ("Jungle Moonkin? What does that do again? OH CRAP!"), and it's makes winning really easy too. Give this an upvote so we can spread the word!