In Final C.A., You should have to pay good concentration in study to pass even a single group. I think if you are giving both the groups then atleast 5 months and if giving a single group then atleast 2 months are required.
According to me, you should have to concentrate more in your study rather than office and do extensive reading and then revision atleast for 3 times before giving exam.
Prefer a detailed study in each subject rather than only important things because each and every subject has it's importance in the practical field also.
So, to pass at the first attemt, hard work with proper manner is a must and there is no substitute for hard work.
Remember the god and work hard and you will surely suceed. ALL THE BEST!!!.