Ghengis Khan enters the midlands with his entire army during the Golden Age Arc, would he be a legitimate threat?
Yes, because the region is still divided during most of the Golden Age and during the latter bit it's still reeling from an all out war with one another. Isn't that one of the reasons why the Kushan swept Midland and its neighbors so badly?
Unless Ghengis' forces are strictly limited to realistic strength and tactics while the fictional forces are not. In that case they're all screwed. If they aren't, though, they probably have their fair share of weird supernatural shit and soldiers with superhuman strength at their disposal.
If Griffith had been female the sexual tension between he and Guts would be thick enough to cut with a knife. It would also make the height of the Eclipse very strange indeed.