Ed, you got it, no easy shortcut. A technique that works well for most subjects is to first start by trying to answer sample questions. You will probably get quite a few wrong - that's okay, it is the window to understanding providing you read up where and why you went wrong.
For plotting, first understand the theory well then focus on speed and accuracy.
You mention magnetism is boring, true. But you will get at least one question worth 3 points (out of 24, so effectively 12.5%) on compass swings. More likely you will get 2 such questions (25%), so you wont pass if you drop these.
Good luck.