Hearthpwn

Basic idea is to draw through as many Cards as possible in the early game and later to reduce as many Combo pieces by emporer as possible.

For the OTK you'll Need at least Spiritsinger Umbra, Baron Rivendare, 2 chargers (just in case your weapons deathrattle hits your only charger first) and a couple more low cost minions to bounce multiple copies of the buff around.

Mulligan:

If you're facing a Control match up, hardmulligan for Divine Favor and Coldlight Oracle and discard all high-cost Cards.

Against aggro, hardmulligan for Solemn Vigil

Play of order of OTK:

1. Equip Val'anyr and attack once but don't activate it's deathrattle in case if you have Spiritsinger Umbra, Coldlight Oracle or Emperor Thaurissan in Hand.

2. Reduce enough keycards for the Combo.

3. If you have all Combo pieces, play Spiritsinger Umbra then attack with the weapon. Now Play Baron Rivendare and whatever minion carries the buff. If it's a charger trade if possible into an enemy minion to proc the deathrattle.

4. Whichever minion has the most buffs now inhand should be played next, expect for if it's your last charge minion. Repeat this as much as you can until you Play your last charge minion to attack your opponents face. 

(if you have another set of pyromancer and equality in Hand an spare mana you can trigger their deathrattles another time)

Enjoy.