I do believe that now a days many people tries to spam low life minions in the early turns. I believe people called that style of play aggro. I also believe that as a mage or a priest the style of playing can't be aggro and is more of what I believe people called control. You can also play a shaman with a control deck. If the mage is not able to put a control on the minions in the early game, then she will suffer from a lot of income damage and no matter if flamestrike clear the board or not, the mage is already weak. The game does not ends when the mage clears the board with flamestrike, I would call it the late game phase just start. Perhaps you should try to include some minions that could be used in the late game phase or perhaps minions that can take the damage of the flamestrike.
In conclusion, don't claim a card is op if you haven't played the game that much and you are not able to distinguish from an aggro deck from a control deck.