300 level classical mechanics usually only encompasses two problems on the PGRE. You may have to set up a lagrangian or hamiltonian, but other than that, the mechanics problems will be at a difficulty of a 100-200 level course.
The PGRE is not about complex problems, but instead about testing how quickly you can solve introductory and modern physics problems. Hence, you are going to need to memorize as many formulas as you possibly can! (Yeah, I know it's stupid but this is the PGRE...)
The best way to figure out what you need to know is to do the released practice PGRE problems over and over again until you can solve each one in under 1.7 minutes! This will help you see what material they ask most often and you will figure out some tricks to help you solve the problems fast!
Remember there is no calculator allowed so you also need to practice rounding, estimating, and doing math in your head if your not used to this.
Your studying should revolve around the released GRE problems, not problems from other sources! Only old GRE problems are going to give you a good representation of what the current test may be like.
Here are some other resources to help you find advice for the PGRE:
physicsgre.com
grephysics.net (answers to old PGRE questions)
http://www.physics.ohio-state.edu/undergrad/ugs_gre.php (has all the PGRE released old tests)
http://onphysicsbooks.blogspot.com/2009/01/studying-for-physics-gre.html (good books to study from)