You can follow below strategies for your preparation

1- Timetable-  You should make a proper time table for your preparation and include both the syllabus for JEE Main and boards exam so that you can complete your syllabus and you get proper time for revision.

You should focus on the topics which carries more weightage in the exam.

2- Focus on theory and concepts- You should clear your concepts to score good marks and to build strong fundamentals. 

3- Previous year papers- You should solve past year problems to analyze the exam pattern and to understand the level of questions.

4- Revision- There are some important  topics in the JEE+Boards syllabus that require students to learn  by rote, like trigonometric identities and  inorganic chemistry- this is where regular revision will help you.  In Board Exams, you are required to write theoretical answers. Regular revision is the key to scoring high  marks in Board and JEE exam.

All the Best!!