You can follow below strategies to crack your exam in 15 days-

Physics-

Make sure your concepts are clear. Make important notes, write down all the important formulae, shortcut and tricks. It will help you to revise a day before the exam. Practice as much as MCQs for your practice. Don't try to attempt complicated questions in a last month.

Chemistry-

You can take help of NCERT book for chemistry. Try to attempt all the questions and exercises which are given at the end of the chapter. You can also solve previous year papers questions as well for your practice and to understand the level of the exam.

You can also refer other books other than NCERT for your preparation but NCERT should be your first priority.

Mathematics-

For Mathematics, you need to practice more and more. As much questions you will solve you will build a command on the subject.