How to go from Chemistry AS grade C to A at A2

Hi,
I got an A in biology at AS, B at history (although my teacher didn't teach us one of the 4 topics that came up sadly so that was annoying) and a C in chemistry which oddly was one of the highest, but still I don't see that as much of a consolation, even though our teacher left after a few months so that wasn't ideal either :/
Basically, I really want to get an A at the end of A2 in chemistry. I am going to resit both AS exams with my 2 A2 exams (I know, it's going to nearly see me off), but I know it can be done, as it has been done before and I'm sure others will do it again.

Has anyone ever done this, and if so (or even if not), do you have any advice on how to do this please? Obviously I want to read around the topic more too this year as I think this will help me mentally collate all the information and make sense of it.. does anyone recommend anything useful to read?

Also (sorry to be a pain), if you have done your A2's, when did you start revising for them, and did you feel prepared enough?

Thank-you :smile: