Japan OP Paradox - Are Damyos an unfair advantage?

So, two friends and I have been loosing to Japan a lot lately on supremacy games. We are focusing on a battle one place and suddenly a wild Damyo appears behind our city, makes samurai, break the walls usually at least 3 layers thick, and rampage through our economy. Most of the time, by the moment when our army has moved from the front to deal with the backstabbing SOBs our economies are destroyed. Last game, we manage to kill the Daimyo but just at the last second before the last hit that killed it, the bastard sent a second Damyo card to the first one and kept running and spawning troops in our almost non-existent city.

I think Japs are OP but maybe manageable, we need either to place more walls even though sometimes is very hard due to space, or change our tactics. One of my friends, let's call it A, says its super OP and basically, if someone plays with Japs is playing with an advantage and most likely will win so he will never play again against a Jap. Note that we are two sergeants and we have a few times beat up lieutenants, we are not that bad. Friend B, also serg, says it's not OP and we just sucked and it's our blame if we lose.

One friendly discussion after on another different subject friend B challenged friend A to a 1v1 duel. Friend A wants to go Japs and says that since friend B says it's not OP then it wouldn't matter and it's fair game. I say that if he goes Jap after saying that it's OP and refuses to play against it it's just hypocrite and if he wins will have no honor since he needed the advantage he claims it gives even if friend B says Japs are not OP. On the other hand, if he does win with Japs, will he be proving that Japs are OP and by choosing them you have almost a certain win? If he looses would it make it more humiliating?

What are your thoughts on this?

And, do you think Japs are really that OP or not? Any advice on playing against them?