This is the deck I used to defeat Heroic Giantfin. The strategy is flooding the board to deal with him doing the same.
Use your AoE spells to help clear out all of his murlocs and keep your board alive. This is key because you need sustained damage to his face to kill him before you get overwhelmed.
If this helps in your adventures, please leave a thumbs up. Good luck!
Edit : This deck is far from perfect, but will hopefully secure you the win after a few attempts. Thank you for all of the constructive comments, I'll keep the deck updated with your improvements.