Hey! I'm also in year 12 and doing chemistry and physics. I got 3 A's for sciences at GCSE, though in all honesty GCSEs' are the worst indicator of A level performance (unless you are got 10A*). I find for Physics, the best strategy is doing questions after question, past paper after past paper, just like maths. If you are doing OCR A, then around 50% of the exam is definitions. For Chemistry, the most important thing is understand the BASICS!! You need to understand the basics which are the fundamental foundation everything you will learn. Also I find the questions in chemistry require you to think more laterally, and there is a broader range of questions that may be asked, though this may be untrue.
I understand your fustration with end of unit tests, which in Physics (being my favourite subject), I am getting Ds.