Hearthpwn

I built this and beat Giantfin Heroic on my first try. His murloc flood can easily overwhelm you, so the key to this deck is efficient board clears. Giantfin tends not to clear your minions, so by repeatedly wiping his minions out you can build up board presence to go face.

Tips:

All minions you have should go face - always use removal spells to deal with his minions, or minions specifically suited to the task, i.e. explosive sheep or dark iron skulker.

Hungry crab is an awesome card, as you can instantly remove any of his minions and you can couple this with shadowstep for a very cheap removal combo. 

If he buffs the health of his minions, use deadly poison or goblin auto-barber to power up your HP blade for a blade flurry

Potential Swaps:

I didn't use Tinker's Oil, it's a little OP for the job at hand, you could cut that for goblin sappers, as he will usually have more than six cards

This deck could suffer from lack of card draw, so again you could cut Tinker's Oil for any kind of card draw

Ironbeak Owls could also be effective if any of his minions get out of control