Best card in arena? Very possibly. That 9/7 kraken is considered one of the best arena cards, and while that's targetted, this does twice the damage (and if you can minimise the board it may as well be targetted).
People who think this is only an arena card just look at Bomb Lobber who is a fair card and saw a little bit of play in GvG. For 3 more mana you get double battlecry and +1/+7
It doesn't have taunt like Primordial Drake which is a big deal, but being able to clear a wide variety of boards without hurting your own minions means you can play twin tyrant in more aggressive midrange decks.
It could also be powerful in quest shaman. You can clear 2 minions with 8 health or 4 minions with 4 health or anything in between. I like big battlecries in quest shaman anyway, I miss my Kalimos.
Seems like people are underrating this card. We'll see, but it seems very strong staple for dragon control type decks. This helps you stick to the board and I'm assuming this will be something that lets you get back or get ahead on the board. I think people are not correctly valuing dealing 8 damage split between 2 minions (equally). It has potential, certainly amazing in arena, to be played in standard for decks that need a higher dragon count and want more access to removal tools. I think this will be especially valuable in highlander style decks.