Hearthpwn

This is the deck I beat / had the most success against Nithogg with. In a perfect scenario you want to be able to put eye of the storm in your Tess pool, along with any other spells that clear the board/ boost your board. Also pick Nozari when you can, as you are likely to need a bit of a health boost. Otherwise use spells and your hero to control the board, and as long as the board is controlled you can win. 

As a side note, I would recommend you craft this deck as cycling through whizzbang can be very tedious, but as a f2p whizzbang is probably the way to go.