Hello,
Now you have only time for revision ..so start revising the things that you had learnt in the last 1 year and now don't study new things.Just practice previous years questions and revise formulas and important concepts.Mostly questions are based on ncert books so read and practice ncert at this moment.Get your all doubts clear from your teachers as it is very important.You can easily get 85% in cbse 12th board if you are consistent with your studies and follows a proper timetable.
Good luck.