Hearthpwn

A 5-mana, 3-damage to all enemy minions spell would probably get played, without a 2/3 body.

Thematically, I also don't like Garona as a board clear.  That doesn't really say "master assassin" to me.

"Deal 4 damage to an enemy minion, then gain stealth" on a 5-mana 2/3 seems fair.  Maybe even 3/3.  Yeah.  It'd compare interestingly with Vilespine Slayer which is a 3/4 which can destroy a minion, but this has stealth and is battlecry instead of combo.  It'd combine the value of a 3/3 stealth for 3, with an Eviscerate that can't go face.  Good tempo which doesn't seem too excessive.

The 10 mana Garona is far too weak. It's too expensive for it's effect. Tyrantus, for example, is a really scary card if you're opponent can't kill you or slow it down. But they usually can. For a 10 mana card to be good it has to have an immediate effect that your opponent cannot just ignore and kill you. Take, for example. Ultimate Infestation. It can clear one of the opponent's minions or hit face for 5, then summon a 5/5 minion that can kill the straglers, gain 5 armor for extra protection, and draw you 5 cards, giving you more options for what to do next turn.

Hope this helps! :)