Just screenshots of where to find these secret bosses.
Secret Boss Guide
There are 9 bosses you need to defeat to unlock the achievement.
Images may contain spoilers.
Basically, you talk to NPC with green (!) mark to unlock the boss and then go to the boss location to beat them.
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Ekurorepyui
Talk to the NPC kid.
Go via boat from Ashiwara.
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Forbidden Scroll
Talk to Mr.Raurau
Open field near Ashiwara.
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Gashanagui
Talk to the NPC in the outpost between Ennakamui and Tokara.
(Location might be incorrect)
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Haramuwa-Nnaki
Talk to Nekone.
Inside Tabari Cave deepest part.
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Kemura-Tokkoro
Talk to the NPC in the outpost near Imperial Capital.
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Martial Artist
Talk to the NPC sitting in front of tea house.
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Paketunashin
Talk to an NPC in Imperial Capital. And you’ll get worried About Roygen.
Get inside and talk to Roygen.
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Saie-Ruke
Talk to the NPC near a stall.
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Sheporo-Imoku
Talk to the NPC soldier, he’ll bring you to Muramasa.
Roaming the desert. Just look for a big worm.
Thanks to The Salt for his great guide, all credit to his effort. you can also read the original guide fromĀ Steam Community. enjoy the game.