The problem here is that deities are bound by the plot, not the rules. If your DM tells you “no spell or weapon of mortal make could ever pierce the Queen of Dragons’ hide”, that means there is nothing you can do to hurt her. End of conversation. But, if your DM tells you “only a blade forged by the divine smith Watsisnaim could slay such a mighty foe”, it’s time for a fetch quest.