Hearthpwn

The premise is the same for this Mage deck as it is for my Hunter deck for the same match-up against Netherspite (in-depth writeup for Hunter deck here).

Strategy: Always play taunt minions directly next to blue portal. Always play support minions (anything other than taunt) next to pink portal. Try to keep no more than three minions on the board at any given time; there will be exceptions to this. The reason is that Netherspite has removal and AOE spells and fewer minions seem to trigger these less often. Sometimes I did four minions on the board, but usually stuck to three. 

I added two Stonesplinter Troggs and two Burly Rockjaw Troggs to this deck because Netherspite casts so many spells they act as great buffs for the Troggs. 

Other cards that would work well in this deck are  Counterspell and Water Elemental.

Mulligans: Look for Glacial Shard and Annoy-o-Tron; that is pretty much the perfect mulligan. Also keep Nat Pagle. Anything other than one- or two-cost cards, throw back. Use Glacial Shard on the pink portal on turn 1 to freeze Netherspite's face and Annoy-o-Tron on turn 2 on the blue portal. 

Note: Use Snowchugger to freeze Netherspite's face, as he cannot attack you when he is frozen.  And since Netherspite's attack is 8 damage to the face minimum, the more you can keep him from attacking you, the longer you can survive.

Strategy: On the windfury side of your board, on your turn, always first attack with the minion you currently have in that slot. That way you get windfury value from that minion. Then lay down a second minion, keeping the minion with the greatest attack in that spot. 

If you prefer, you can run this deck with 

+1 Bluegill Warrior  +2 Argent Horserider  +1 Counterspell

- 2 Stonesplinter Trogg - 1 Burly Rockjaw Trogg -1 Vicious Fledgling

Enjoy!