Way too strong. In most cases this will kill a players entire board. Reduce the damage to 2, and maybe up the cost to 4 mana. If you wanna add some flavor, maybe have it deal damage to a number of random targets. Maybe: When Polly, Esplosiphile attacks, choose heads or tails. For each creature defending player controls, flip a coin. For each creature that landed on the chosen result, Polly deals 2 damage .to that creature."

I like it, thanks for the suggestion!