Oh my lordy, where do I start with this one.
Chromaggus is EASLY the hardest heroic boss i've have ever played. I figured since I couldn't find a good deck to use in the year of the mammoth, i'd make my own.
This deck is a variant of the OTK priest deck that is usually put against heroic chro. The only difference is you can not fit inner fire into the deck. Instead of targeting big minions with divine spirit, we use Lightspawn.
Other cards like Hozen Healer and Binding Heal. Are used to keep your burgly Lightspawn alive.
This is what a winning mulligan looks like.
This opening hand was my first big win against the guy.
You need a or both Radiant Elemental's in your opening hand to win. If you don't draw one by turn 2 restart.
Any early game cards will be good enough as long as you have an elemental.
Make sure by turn 9 you have a big Lightspawn on board and Alexstrasza in hand. Once you mostly have board control, fully heal your Lightspawn, play Alexstrasza, and slam face for 10+. There is no way you're getting through all 60+ health without Alex.
In order of importance to chromaggus, Blue, Black, Red, All the rest.
Hope this helps, have a gooooooood day.