Grasping the main concepts will get you a really solid score, but there's no replacement for exact terminology, definitions, specifics etc for getting a really high score.
Once you understand the general concepts it comes down to doing the tedious slog work of memorising. Write out the same answers over and over again; recite them over and over again; look-cover-repeat-check over and over again - I know it sucks, but you need a lot of exact word-perfect content in Legal Studies... and, as a bonus, your writing speed increases the more memorised the content is, so you're getting something extra out of it!
Plus, the more memorised your key definitions and facts are, the less chance there is of you missing something out or expressing your knowledge in an ambiguous/vague/wordy way.