To score above 200 in jee mains 2018 you should focus on chemistry as it is very much scoring and consumes less time if you get a good command on chemistry you can score even 100 marks in chemistry. And for mathematics go through each and every topic and give a good attention on calculus,co ordinate geometry, vectors. And for physics as far as previous year papers are concerned level was pretty tough despite of paper of 2018 so don't much rely on physics .