To crack IIT-JEE you can refer below points-

1- You should refer NCERT books for your preparation. Make sure your basics are clear. Don't use much reference books. You can use good recommended books for your preparation.

2- You can use previous year papers to understand the exam pattern and level of questions.

3- You can solve as many as MCQs for your practice and to build up speed and accuracy. You should solve short-answer questions, subjective-type problems and questions based on assertion reasoning to develop deep understanding of the subject.

4-Important topics you should cover for JEE Advanced are:

    Physics: Mechanics and Electrodynamics are the two most important topics but one should also devote time to Heat, Thermodynamics, Optics and Modern Physics.

    Chemistry: General principles of Organic Chemistry are easy to understand. Numericals of Physical Chemistry are scoring. Inorganic Chemistry has many facts and figures and requires constant revision.

    Mathematics: Master inter-relationship between various topics. Coordinate Geometry and Vectors can help one to solve problems related to complex numbers. Permutation and Combinations help you to solve problems of Probability. Trigonometry and Calculus again help one to solve all types of problems.

Below is the JEE Main expected cut off to qualify for JEE Advanced

General- 95-110
OBC- 70-75
SC-50-55
ST- 35-50

There is no challenging question as per now in JEE Main.