How strong do you think the Six Great Nations at the centre of the continent are?
I think the Six great nations would be the ones to form the union of allied countries against Nazarick.
Most of these countries are probably stronger than all the human nations shown so far apart from the Slane Theocracy. I believe three (3) of these countries ate humans as a food source like the Beastman country that is attacking the dragon kingdom.
No human nations are even considered as none of them are in the centre of the continent.. But at a guess The Minotaur nation, the Beastman country and the Troll Kingdom might be among those six.
Maruyama have hinted that there will be an alliance of the big demi-human nations against Nazarick earlier and that war will include most of Nazaricks resources like the floor guardians, even Gargantua will have to be used to stop their Golem centaur cavalry we where told.