"This girl is a hard nut to crack."
Translation:Ця дівчина - міцний горішок.
I guess it is assumed that you know the situations in which the phrase is used in English :)
The situations in which the person is tough and strong. E.g. A professor is "torturing" someone at an exam, giving them all kinds of nasty and tricky questions, but the student answers without problems and does not get stressed.