From what I’ve read online this badge is obtained by defeating all 23 different monsters. I completed this back when the bestiary list was much longer but only 23 monsters were in the game. I can see other people have earned it by looking at the corresponding PS5 trophy. I’ve tried killing a couple more monsters to add to my kill count to see if the causes the badge to activate but this didn’t work.
#B.O.S.’s Finest Badge bug
tacit hill
rapid thunder
Hello @tacit hill. Thanks for reaching out. We are aware of this issue for some players who have completed the badges before it was fixed, and we're working on it now 