Hearthpwn

The plan is use your early and mid game cards to control the board, then use Alexstrasza to lower your opponent's hp, then make the finishers with either Zephrys the Great, Dragonqueen Alexstrasza, Dragoncaster Power of Creation, Kalecgos, Puzzle Box of Yogg-Saron and Pyroblast.

Card Guide:

Arcane Breath - Good early game control, and can be evolved into more control if you're holding a dragon

Ray of Frost - Early game control

Doomsayer - Combo with Frost Nova to destroy entire board on your next turn or could just be played on an empty board so that your opponent doesn't play any minions

Dragon Breeder - Not used as an early game card, but as an extra value card. This card is insane value with any dragon, including every single dragon in this deck

Firetree Witchdoctor - Good value card and possibly you can discover a control spell

Frostbolt - Early game control

Zephyrs the Great - Either use him to clear the board or to finish an OTK with Alexstrasza

Arcane Intellect - Card draw for more control and OTK

Counterspell - Just a generally good secret, when used right it's just great value

Flame Ward - Early game control

Frost Nova - Combo with Doomsayer or just freezing a board which is sometimes really good

Azure Explorer - Spell damage +2 can come in handy when setting up a bigger board clear with Rolling Fireball or Blizzard and it discovers a dragon, a great value generator

Fireball - Used for OTK but can be used to clear a big minion, you can replace the missing Fireball by discovering it with Zephrys the Great

Scaleworm - Early game control card if you're holding a dragon

Big Ol' Whelp - Card draw and a really good dragon

Malygos, Aspect of Magic - A good mage card in general, could give you board clear, card draw, singe target removal, very good control card. If you do get an Upgraded Fireball (4 mana deals 8 damage), you can use the OTK of Alexstrasza into Fireball, Upgraded Fireball and hero power so it can be used for OTK aswell

Rolling Fireball - very flexible card, can be used for a small board removal or a single target big removal

Zilliax - Control and healing

Blizzard - Control the board, a very greedy but awesome card ;)

Dragoncaster - OTK with Pyroblast

Reno the Relicologist - Control

Crowd Roaster - Control

Siamat - Very flexible card, similar to Zilliax, but is purely used for Control (and is better at it)

Power of Creation - Big spell with Big value

Tortollan Pilgrim - Big value if you get a high cost spell and RNGesus is on your side

Alexstrasza - OTK, the most important part of it

Dragonqueen Alexstrasza - Great value with dragons and a very giant minion that has to be cleared otherwise your opponent is going to have a bad time

Kalecgos - Great value with any spell in your hand or one that you get, it's a big threat for your opponent

Puzzle Box of Yogg-Saron - PURE RNG, can kill you, can save you, use in a sticky situtation

Pyroblast - OTK with Dragoncaster

Mulligan:

Against aggro and midrange - Go for control cards, take advantage of the early and mid game, and in the late game finish them off with Dragonqueen Alexstrasza or when you draw OTK, do the OTK

Control - Hard mulligan for OTK, you need to draw it as fast as possible because most of your control cards are not as useful, and because warrior's armor is a pain in the ass for Alexstrasza. If there is a control warrior meta, which I do not believe that CW will be alive at all, switch out a Ray of Frost for Platebreaker