That's what really sucks about physics, you have to focus on EVERYTHING because you never know what will show up. For me I had trouble trying to derive all of the kinematics formulas so I wrote down all of the different forms. I also struggled with the electrical forces and magnetism so I made sure to write those down in an easy to understand way. The FL's I did were very circuit heavy (ohms law stuff) so I focused on those equations but both of my actual MCAT's had VERY LITTLE circuit problems. My best advice is to practice the ones you struggle with but generally have basic knowledge of every concept and equations that go with them.