If u have problems in costing subject , it is better to attend the reputed classes nearby your residence having good professors and clear your doubts till you are confident about the subject .This is because unless u are not confident about the subject and it's preparation u can't go into the exams with confidence .
In my opinion Best Book for costing is PADHUKA's . Other good reference books for costing may be found in this site .
As far as giving both attempts or one , it's upto you , If u r fully prepared with both the groups then u should go for both the groups , because when u give both groups chances of set - off are there .
My advise will be to prepare for both the groups(atleast three aubjects in each group perfectly ) and give both the groups .
Preparation for Subjects :-
For each subjects mark out the important topics and read it thoroughly and revise it atleast for two times . For this u have to make a daily study schedule and see to it that adequate time is given for each subject and it's topics .
Give more time to the topic which is difficult and important .
Try to stick to this schedule and also give adequate time for revision, as it helps the mind to register the things which u have already studied and makes u more confident before going for exams .
Also, try to write past exam papers for theory subjects like LAW , AUDIT & I.S.C.A and compare ur answers with the suggested answers of ICAI . This will help u to know what the ICAI expects from us in theory paper and u can accordingly adjust your paper writing skills .
BEST OF LUCK ........