Hearthpwn

This Deck consists of basic cards and is more dependent on value trades and playing the tactics, instead of fancy card effects and AI abuse.

I did beat Nefarian Normal in the second go,but it could take a few more tries.

Mulligan hard to get at least one Elven Archer into your starting hand.

Dont be afraid to concede and restart if you dont get any 1 or 2 drops early on, that's also why I lost the fist attempt on this boss. He will flood the board with a decent amount of 2/1 minions and trade into any of your minions which has 2 or more attack power.

It's also important to delay phase two until you get a [card]Acidic Swamp Ooze[/card], so you can destroy Onyxia's weapon immediately. Do not destroy Nefarian's armor till you one of the two Oozes on your hand.

In phase two you have to keep your board minimalistic. Nefarian will attack your face with 1-4 Fireballs each turn, which deal 1 damage each fireball. Animated Armor or Taunt has no effect to prevent or lower the incoming damage from this ability, but Ice Barrier works well and can prevent face damage by Nefarian for up to 4 turns. Onyxia also tends to  use LAVA! if her board is completely empty.

Again, keep your board minimalistic since Nefarian kills your whole board as soon you reduce Onyxia to zero health. However, his whole board is not affected at all! It kills everything, even legendaries and divine shield minions. That's the point where Flame Strike becomes pretty powerfull. If possible, keep your Ogres, Warlords and Champions for phase 3.  It's no drama if you had to use one or two in phase 2. 

Play with patience. Do not try to rush the boss down.
It's more important to prevent damage, than to deal damage.