Demon Duster achievement

[Suno]

So I'm doing a 100% run and I've gotten every achievement save for time attack, beating the game on Nightmare and Demon Duster(because I don't wanna finish it until I get Demon Duster).
I'm only missing one monster in the Beastiary and they're the Zearots(AKA the Gatekeepers).
Problem is I've killed them over and over, and I can see all of their info on the right side of my screen(how much EXP or gold they give you, their attack, all that good stuff) and yet they don't show up on the beastiary, the only two entries missing are them and the final boss, so what gives?
Is it just bugged? Should I give up on this achievement? I perfected YS1 so I'd really like to do the same for 2...