Don't get me wrong here, I don't want to completely rule out the possibility that it is entirely unachievable. However, given the fact that CAIE Further Mathematics - 9618, has 4 components: Further Pure Mathematics 1, Further Probability and Statistics, Further Pure Mathematics 2 and Further Mechanics... I really think it's not a good idea to cram and do all of it in a year considering the fact that you haven't finished A Level math yet. Again, it all comes down to you... because I personally did Further math alongside my AS level math (not even AFTER doing my AS math), and although Further Pure Math was easy enough, I struggled with Further Statistics because I hadn't done **** in my AS Probability and Statistics 1 course yet. Still, I did put my heart and soul into it, and achieved a high A grade for AS level and I'm pretty sure that if I work hard enough again this year, I'll get that A* in A level. It's just that, you'll have to put in more effort than your peers in Further Math class and have to finish A level math on your own before you start because some chapters like Vectors and Continuous Random Variables (Further Stats) carry forward the concept introduced in A Level math. So that's pretty much it, whatever you end up decide doing, good luck with that!