So, in a world of extremely low level magic (by fantasy standards, and the staple of all fantasy, D&D) we have now been introduced to a character who can raise a literal army from the dead in seconds, with no limit on numbers, and not even as much as a ritual needed to do so.
If this were a video game or pen and paper, that guy would be banned for hacking or kicked out of the group for cheating, because that's not stuff gods do in most fantasy.
I truly see no way that humans can win this war unless we find out that the only reason they are marching on Westeros and the world in general is because they think all dragons are dead and dragons are the one thing that can stop them (or, ya know, make an army's worth of dragon glass weapons and such.) and this is how we get the long shipped marriage of Jon Snow and the Khaleesi.
I think a fair thing to throw out is this. Let's say Snow hurries to King's Landing with all his men and thousands of wildling refugees and gets a seat with the court and with the high sparrow all at once and tells them what's happening. Even with a thousand witnesses, do they believe? With Tommen as king, do they do anything? Even if Tywin was king does anything happen? This truly feels like a completely hopeless scenario.