Hello! Thank you for the question. Sorry you've not been able to find an answer to this. I'm unable to find in the book where it says monsters are not impacted by wounds. Would you mind pointing out where that is mentioned, please? Regardless, I can understand the lack of clarity.
If you are playing the game, I say this: On a success, you do 1 Harm, reducing the creature's rank by 1 step. Same as with a PC.
There are a few clarity issues in the book, that I intend to adjust in the future.
Please reach out if this doesn't do the trick or if there are other questions.
All best,PR