Ok here's a guide tested by me in this game and feudalism 2
First, start off as an far east person
up your DEx until you can wield some pretty good armor and weaponds, by doing quests.
Now, conquer all of the far east empire, just to get started
Once you conquer nagoya, you should be able to wield the 2 best ninja tos and the best armor.
(I recommend using ninja tos and spamming the last dual weapons attack)
For the OHKO guy, just take off all your armor and buy a ton of 300 health potions.
Once you have conquered Nagoya, build up your army to max size and head to any capital.
Conquer it, kill the champion after buying like 4000 potions to last you the whole game.
Now rebuild your army and head to the next capital rinse and repeat
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