Hearthpwn

Giantfin's deck is all murlocs, and a broken special spell that summons 5 murlocs that died during the game.  His hero power draws him 2 cards per turn for free.  You need a deck that can handle the murloc onslaught.  This deck has a ton of different ways to clear a board of small creatures before they get out of hand.

Your win condition is to clear his board and get both of your Murloc Knight out.  Then, you'll get the equivalent of his hero power every time you use yours, and you'll be able to control the board with minions.

Getting a Divine Favor early helps a lot, because his hero power fills his hand up at the start of the game, but he's able to play more cards later on. Hungry Crab [/card]is also a nice thing to get in your opening hand, and will often trade 3 for 1, negating a turn of his hero power early in the game, and making it much easier to survive until you can get [card]Mysterious Challenger and Murloc Knight on the board.