I am flexible. In the past students have come to me with problems that we solve together, and also I have taught to exam specifications.
I employ a "retrospective" revision type of approach where we assess the competency of the student in crucial areas, and on performing a "diagnostic" wherein every area is tested, we can target the ones where the candidate lacks.

This seems simple; obviously one'd target where they are weak, but as Socrates said "I know that I know nothing" and he was pretty wise; I think his message is that in order to have "some" idea of what we know, we need only know the complement - what we don't know.