No one can tell you main topics which you should study....just focus at your syllabus and solve questions....give mock tests...analyse your performance and your weakest area....then work on that area....be motivated...all the best!

There is nomain tooic as such which can guarantee you sure 150plus scores.
All you need is to focus on complete preparation by January, Feb and March should be devoted for mock exams.
You should then focus on weak areas and recrify mistakes by april and revise again so to avoid such repetitively.
Make sure you keep revising basics.
Giving more than 20 mock exams will sureshot increase your chances for 150+.