There is no topic that is less important. Try to study every topic as you don't know from where the question may come in the exam. Stay prepared to your best so that you can clear the exams easily. But if you want to know the important topics, then check the practice Manual. in it all the important topics are given, you can easily prepare form the practice manual.
Hello, One thing you need to consider is that all the topic is important in the costing and some topic on which more concentration is need such as Transfer pricing, Decision making, CVP Analysis and other. But to get it you can refer the trend of the question papers of last few years