Your study method should depend on your goal.
For example, if your goal in an exam is to just pass, you can study all the easy chapters and leave a few difficult sections. Spend time on what you are good at. You will do well.
If your goal is to get the top rank, of course, you have to study everything in the course. Even in this case, if the question papers give you a choice, then it is OK to leave a few difficult sections.
Similarly for a competition, find out the exact pattern of the question papers. Is there choice of questions to answer? Find out approximately what percentage marks students need to succeed. It could be as low as 40% in some competitions and as high as 95% in others.
This is a difficult decision to make. Discuss these ideas with some friends. Finally, talk to past students who have already achieved what you want to achieve. It may also help to discuss this with your teachers because they may have guided students to success in the past.
In any case, revision is extremely important for both and so are speed reading, memory maps or mind maps.