(hard|tough) nut (to crack|)

someone or something that is difficult to understand, deal with, or overcome

The idiom "hard nut" is used to describe a person who is difficult to deal with or a problem that is challenging to solve. It originated in the late 19th century and draws its metaphorical meaning from the idea of cracking open a nut. Just as some nuts have tough shells that are hard to break, this expression refers to someone who is resistant, unyielding, or stubborn.

The negotiator encountered a hard nut during the business deal, as the opposing party refused to compromise on any terms.
Susan is known as a tough nut to crack when it comes to her emotions.It's difficult to get her to open up and share her feelings.
Solving the puzzle proved to be a tough nut for the participants of the competition, as it required intricate problem-solving skills.
The detective found the case to be a hard nut to crack due to the lack of evidence and unreliable witnesses.
The new math concept was a tough nut for the students to grasp, requiring them to put in extra effort and seek additional help from the teacher.