Gurumin: A Monstrous Adventure

Have you used Jason Venter's walkthrough to see in which area you're missing enemies?
That's my suggestion.

there is a link to a pretty good walk through. it talks about how to get an S rank in every stage. that should hopefully help you.

www.gamefaqs.com/portable/psp/file/932300/47997

Okay, I'm still playing this game and just finished Forgotten Den. There are 3, i think, water-based enemies in this area.
There is a secret passage/area- -how to get there- if you drill the weak wall behind the treasure chest, after fighting with the 2 enemies guarding it, you can drill the crumbling matter after the wall, keep drilling through 3 or 4 layers of the stuff until you get to the secret area. As you move around there you will see several enemies above you; don't worry, they'll come to you- -2 at a time. These are just some ideas I have as to where you may be missing enemies.
I hope this helps!
Sincerely,
w/o

I just realized a few minutes ago that part of my earlier answer is incorrect. When Parin steps into or rather onto the mound for the drill power-up, the enemies above will descend, 2 at a time until the final 1 or 2 are defeated. I hope this helps. This is a several-step stage of the game.
Sincerely,
w/o