How do I get a A in biology A-level?

I got a C last year too, but we did a mock in December in which I got an A in.
From my experience, what I did wrong at AS is that I focused too much on revising the material and not putting enough effort into practicing exam technique. As boring as past papers sound, if you do enough of them it makes things a lot easier. Papers have repeated (or had very similar) questions before, which means if you're not new to the question it helps great.