Hearthpwn

Starting Hand:

You want low cost damage spells. Ideal hand would be Mirror Image, Frostbolt, & Frostnova because those cards will give you some time to build up your own minion army from Guzzler's hero spell.

Strategy:

Pretty straightforward, other than the control spells, just lob in your strongest minions. Battlecry creatures will likely not be played so if you got beefy Deathrattle minions thats the way to go! 

I didnt have sylvanas back when I did this, but if I did I'd add sylvanas instantly for obvious reasons.

Tips to beat Grim Guzzler on Heroic difficulty :
1) You don't need my specific deck, the basic idea is have the strongest minion cards in your deck. If you draw them as beginning cards, make sure to recycle them in hopes to get low cost mana spells.
2) The strategy using a mage is to use spells like Ice Lance & Frostbolt to delay or even kill enemies before things get out of hand.
3) Whenever its possible to kill an enemy creature without having your attacking minion die, do it. Its worth it to get rid of them. The Force Tank MAX works best in this scenario.
4) Remember that you dont expect to actually play any of your heavy hitting monsters, Grim Guzzler will likely play them for you so you want to make sure every minion you have is a hard hitting one.
5) When you do have a large amount of minions, he Grim Guzzler will use BRAWL. (http://hearthstone.gamepedia.com/Brawl). This is a huge luck factor to win this unless of course he plays Dark Iron Bounce (http://hearthstone.gamepedia.com/Dark...) which he may have. If this is the case, make sure to polymorph or kill Dark Iron Bouncer ASAP, otherwise you will lose every brawl.
6) Remember its still very lucky to have the correct order of your minions auto spawned so you definitely need to try a few times. Just make sure to have control spells to delay death or enemy minion actions.

Youtube Link : https://youtu.be/vZVFeRFTALM