If you are in class 12th than my suggestion for you is that start preparing for boards first as cracking JEE in a such a sort period of time is difficult without prior preparation.Cracking JEE require lots of constant effort with consistancy.Here in your case there are more number of chapter for prepare than number of days left for the exam. You have second chance for give this exam than try it for that time.Boards are going to be more important than jee.

Try for the second chance for JEE with preparation is best way to crack JEE.

Hope this helps you.If any queries left than comment below.