DISCLAIMER: Since People are complaining I'm gonna write this above the deck's strategy, this deck is MULLIGAN DEPENDANT beacause it's a cheap deck I made to help f2p players that can't afford expensive lists, if you have access to all the cards, search another list and don't complain in the comment section. I want to help people, making a 20k dust list doesn't help anyone! That said, here's the strategy:
Mulligan for at least 1 Lightwell and try to survive early game, in this phase you can use an Inner Fire on Deathlord . Once you have board control, choose the minion you want to use to oneshot KTZ and buff/protect him, he doesn't have single target removals for high health minions. At this point due to the combo with your Northshire Cleric you can draw your whole deck by healing the minion you choosed (Check the video), Circle of Healing will help you too if you manage to free trade some minions. Don't destroy all of his armor or he'll go second phase early. Note that if he uses Twisting Nether you have to reset. When your minion is fat enough, use Inner Fire and attack to oneshot the poor Kel'Thuzad. REMEMBER: The combo can change depending on the minion you manage to buff.
NB: My PvE lists must be accessible to everyone, so I try to make them cheap and efficient.
As I promise here's the video:
(My mic didn't work at some point, so I cut it off completely)