How should I study for the JEE Main 2018 in the last 10-15 days to get 200 marks ?

If you've been preparing for the past 10-14 months, I'd advise you dive into solving sample papers right away. Solve JEE Main/AIEEE papers from the last 10-15 years, one every day until the day of the exam. See which topic you're finding difficulties and use the rest of the day to hone that particular section. If you're just starting, 200 might be too steep a target. In your case, my advice would be to study the topics which carry the bulk of the marks. So coordinate geometry will be a good start. Calculus should also be on your radar. In Chemistry, refer NCERT for everything. You won't have time to consult other sources. For Physics, stick to NCERT. Focus more on class 12 syllabus as it carries higher weightage.