Do you think he could get some helps from the magic users that live in Elfhelm? I mean, if the recent chapters taught us something, is that imaginary beings don't always agree with each other, and Guts may have some powerful allies on his battles against Griffith and the God Hand. So, maybe, with a decent army of magic users, he could damage one member of the God Hand at a time, but it's kinda hard to tell since we don't know the true extent of their power in this new world.
At any rate, we do know that his sword has some power to damage godly beings. And if that doesn't work, well, he'll probably make that Schierke line literally true: "No matter how strong, for a human to fight a monster means he has submerged his humanity and transformed himself into a greater monster."