Is the flight planning excercise only for those who have flying experience or do candidates with no flying experience also have to conduct the same excercise?
Everyone does the same flight planning exercise, regardless of any (or lack of) flying experience.
My experience from a previous attempt at the flight plan is always to be careful with your calculations, and NEVER ignore your partner even if you are solving your own problems. Keeping cool is also important. I don't know if they still do this, but back in my time the recruiter would give us a limited time to solve a certain problem. The time does appear to be quite limited, and is designed to give you stress. It did give me a bit of pressure, but in hindsight I figured that the time they give is more than enough, if you can keep your cool.
Good luck!