I made this deck today and beat Chromaggus on the fifth try. I used the decks other people created to no avail. I tried all the priest decks and alarm-o-bot decks yesterday, playing for hours and got nowhere. At first I changed the Alarm-o-bot deck by adding some cheap poison taunts, but that alone wasn't enough, then I thought why aren't I using Astral Communion? Not only will it get me to ten mana on turn four, but it also discards all those annoying Chromaggus spells. Then I thought why not add the druid quest too since it has perfect synergy.
I put those in, took out all other spells which are completely unneeded and very bad to draw after you cast AC, took out most of the eight and nine mana minions, replacing them with seven mana ones, so that you have three mana left over to get rid of the Chromaggus card you get every turn (which you really need to do). And it only took me five tries until I finally got a starting hand with a couple cheap poison minions, and a turn four Astral Communion. It worked perfectly. Technically I didn't need the quest, it did not help me win, even though I did cast Barnabus the Stomper.
Anyway here is the basic idea:
Mulligan for a cheap poison minion, and or Astral Communion. Play the quest. Play the minion. Kill Chromaggus' dude with it, cast Astral Communion. And you are pretty much good to go. The only problems are if Chromaggus somehow kills your poison minion and builds a huge board on the turn you Astral Communion. You could also somehow take too much damage and get killed by spell damage since Chromaggus does run cards that deal four damage, like Swipe.
Don't get discouraged if you don't win right away, it will take some tries before you get the early poison minions and an Astral Communion turn four. But if you get that, and the board is clear when you cast AC, you have a really good chance of winning. Good luck.