Hey AANIS,

Since you will be sitting for 12th Boards and JEE Mains both, you should first cover the common syllabus and make it your strength. This will help you perform well both in Boards and JEE. Make sure that you cover these topics properly - such that you don't miss out on questions asked on syllabus of 12th. If you do this much with the limited time you have - you will eventually improve your rank many times. Don't waste time in covering many topics. Make sure that your concepts are strong in the topics you choose to cover. 

Try mocks to evaluate your preparation - this will help you give the best possible shot.

All the Best!