Dear Archana, talking about NSO, the level of questions is above average. It's not true for every question. More than 75% questions are from your syllabus. Rest  are the HOT questions which require in depth knowledge of your syllabus.

Understanding the questions of higher level is not that tough, you can borrow or download previous year question papers and check the questions, just mark those questions whose level you think is more than average. Now you have a question bank, start preparing those topics, first clear the basics,next try to solve more questions from other books. You can find many competitive level questions from one topic in books or online.

Practice is the key. If you have enough practice then you will be able to solve the problems more quickly and since you have practiced alot from those topics understanding the concept won't be that tough.

All the best

Hope this helps