Way back in Season 1 she tells Joffrey that no foreign army can hold the North. And I'd imagine that no foreign country can hold the North especially during the winter.

I think things go sideways for Cersei as Tyrion is able to out-manuver her. It will probably have something to do with the Iron Bank. Tyrion likely convinces them to back Dany. And then I'm guessing the Gold Cloaks break their contract for the first time ever. And bring in the war elephants to take King's Landing.