This is basically a slightly worse (better with a few other cards) Assassinate, but FREE. There is almost no way to balance a minion like this. Maybe make it 4-5 mana? Even 4 is undercoated, but 5 is overcosted. See the problem?
Still broken. Try comparing it to a card like blizzard or holy nova, both of which see play in competitive decks.
Make it deal 3 damage to ALL minions, and then it will be a very good card, but not broken anymore.
Always remember the "all other minions" instead of "all minions" which would make Garona damage herself. Also, Garona feels like she should be part of the Whispers of the Old Gods expansion instead of Classic just because she is controlled by Cho'gall, a minion of the Old Gods.