Hard work and smart study is the way to go. I didn’t specifically prepare for JIPMER, AIIMS or PGI. Prepare subject-wise to ace an exam. Do not focus on a particular exam as then you’ll be stuck in solving repeats. Do solve repeats, but do that subject-wise, after completing that particular topic/subject.
Most students who crack these entrances are average students with a good preparation strategy. Analyze your weak subjects. With a basic foundation from your MBBS, four months of intense preparation is enough. Do join test series like Dams and ADR Plexus.
During my preparation days, I started my studies by solving MCQs on topics which I had covered the previous day. This was followed by 6-12 hours of study. Allot 60% of your day for strengthening your subjects, and 40% for solving MCQs. If possible, attend Dr. Thameem’s classes. Pathology is a scoring subject that also covers a lot of other subjects. For Patho, attend Devesh Mishra's classes.
I would recommend first knowing your strengths and weaknesses. Polish your strengths further, and improve on your weak areas.