Does being in the top batch in FIITJEE guarantee a selection in IIT-JEE?

Not in a million years.

FIITJEE is simply a coaching institute. Definitely, they are good, but like any other coaching institute, they cannot guarantee a selection in IIT-JEE.

FIITJEE has some fantastic benefits:

1. The overall student pool is large: therefore, one gets a highly competitive environment. It helps in benchmarking oneself to a large number of students across multiple centers.
2. The classroom/batch size is small: therefore, one can get personalized attention. I was at FIITJEE Kanpur from 2011 to 2013 for my JEE preparation and the best thing was that the batch had not more than 35 students. There are some other institutes in the city that have batches of size larger than 1000 students. Yes, you read it correctly - batches larger than 1000 students. The teachers in such institutes teach in an auditorium via a mic.
3. The study material is quite good: I have seen the study material of multiple institutes including Resonance, Bansal, Allen, etc. Undoubtedly, FIITJEE’s content/material is one of the best and most comprehensive. I especially like the GMP which contains a bunch of problems, many of which are above JEE level. It is super helpful for folks aiming for a top 500 rank.
4. Test Series is highly competitive: FIITJEE test series is known to be notoriously difficult. That might be bad for a lot of people, but personally, for me, I feel that if I had the potential of being a top 100 ranker, it was FIITJEE AITS that helped me get a top 50 rank. FIITJEE AITS was super difficult and I also scored average (in absolute terms). So, I pushed myself to score better and solve tougher problems and that helped me do better in JEE.

Having said that, there are some things that FIITJEE cannot offer:

1. No guarantee of selection, irrespective of the batch. No coaching institute can provide this.
2. Some of the teachers in your specific center may not be that great and you pretty much cannot do much about it.
3. Like any other coaching institute, FIITJEE is a for-profit organization. Inherently, all such institutes are designed to attract a large mass of students, most of whom will not get selected for JEE. So, if you have the potential of achieving a top 100 rank, you’d have to go beyond the FIITJEE teachers and make your own way. The teachers are definitely quite capable, but a top 100 rank in JEE Advanced requires an altogether different frame of mind and a different level of effort.

My final comments:

1. Focus on self-study.
2. Do not rely on the teachers of coaching, be it FIITJEE, be it any other. At the end of the day, it is your career. Whether or not you get selected for JEE, the coaching institute will make money via the fee. Consider your teachers are guard rails who will show you the track/direction but don’t forget that you are the driver of the train, not your teacher.
3. Work hard. There is no substitute for hard work. There are no shortcuts to success in JEE. You have to do all the boring things - read books, make notes, solve a lot of problems, repeat, repeat and repeat.