I still don't understand how they can hope to beat Griffith, he's literally a god and practically omnipotent
I agree, but that has always been a thing in Berserk. Guts did not care about the difference in raw power, skill and determination have long been the defining thing that makes him able to defeat the apostles which overpower even him, who in strength is probably the strongest human. It has been how the story goes even since the black swordsman arc. Guts is going "against causality" and defying the power of the god hand by living on even while bearing the sacrificial brand, I think in the endless suffering of Berserk the message of fighting back despite the struggle and the unsurmountable toll it takes has long been a strong point, honestly one of my favourite parts.
Before Griffith's rise, the Kushan Empire was the strongest power in the world of Berserk, and it seems they are continuing to hold their state even with the chaos of the fantastic being brought to the same plane of reality as humanity, and they anticipate that Fantasia is only going to expand, their plan is to be prepared before Griffith decides to finally attack. They are preparing for the inevitable and they will not surrender and submit. In a way, the Kushan Empire after Ganishka's death and the upheaval of the supernatural is preparing to fight on just like Guts.
I fucking love this Manga, RIP Miura, his friends are doing good work continuing with the story he left us.