Good quesion. What is most important for you to score high? 12th board OR nda entrance. Prioritize your schedule. Then prepare according to the priority. There are two way to prepare for any exams. Start easy to moderate to hard. And Start with Hard then moderate then easy. So, you will choose second path, so it will reduce your time.
Watch Book summary of Eat That Frog. It will definetely help you.