See, select 10-15 best chapters in exam in each subject. And do them very properly that you are able to solve it's questions anyhow. So that you can ensure your approximate 12 questions to be correct in each subject which gives you around 144 marks. And also make sure, that you do other chapters too up-to a premature level, so if easy questions come from them, you're able to solve them and can expect 160+ marks.

For chemistry I can guide you by saying ki make one section your strength out of the 3 - Physical, Inorganic and Organic... In physics, prepare well chapters like Friction, Oscillations, modern physics, etc.

Best wishes. Thank-you.