Coaching is necessary just to understand the methodology of how to answer the battery of questions within the limited time span of 1 hour 20 minutes for each subject. The more you practice and understand the best way of cracking those exams, the higher your chances of success. There are many guide books available. It is based on how much you practice. If you are capable of doing the above by yourself it is very good. Most of the students won't take such risks as they are not confident well enough about self study. Otherwise you may take up short-term crash course for the entrance tests.
These are the 5 things to remember to crack entrance exams