In my mock exam in biology and chemistry I got an E and a D I ended up finishing AS with A grades in both. My advice would be go through the specification and look at what you do know and what you don't know. Then what you do is you go through everything that you don't know slowly over the next few weeks I used to do an hour on a particular subject a day. One way of revising would be to make flashcards that have quite some detail on them but are also condensed as in the process you'll end up retaining the information as you make the flashcards. Also do past papers the Biology and Chemistry specs have not changed that much so you can still use the old spec papers for practise, always look at reasons you're not picking up marks on particular questions, if it's knowledge then revise that topic if it's the way you wrote it then look at the mark scheme points and try to structure your answers that way so you don't end up waffling especially on extended writing questions. When revising unless you're doing a paper revise in 20-30 minute periods with a 5 minute break in between so if you had an hour on biology you'd have 2-3 periods of 20-30 minutes with a break in between each one.