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.
Talk to the NPC kid.
Go via boat from Ashiwara.
Talk to Mr.Raurau
Open field near Ashiwara.
Talk to the NPC in the outpost between Ennakamui and Tokara.
(Location might be incorrect)
Talk to Nekone.
Inside Tabari Cave deepest part.
Talk to the NPC in the outpost near Imperial Capital.
Talk to the NPC sitting in front of tea house.
Talk to an NPC in Imperial Capital. And you’ll get worried About Roygen.
Get inside and talk to Roygen.
Talk to the NPC near a stall.
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 from Steam Community. enjoy the game.