I guess it would work against AI3.
But it wouldn't be very effective against another rush deck.
1st turn: Play Harpie
2nd turn: Use it's ability+Heavy armor
3rd turn: Use it's ability.
Most rush decks I know can easily deal 12 damage in 2-3 turns.. and there's also a chance of not getting enough quanta or heavy armor in time for your opponent to be able to kill your Harpie when it's pulled.
And also, since it only has 1hp during the first turn it's on the field, pretty much every single CC card can kill it xD It's fragile..