Hi there.

There is no doubt in the fact that one requires a certain level of intelligence to clear and get the topmost ranks in AIIMS and other such competitive examinations, but yet again, one can't agree enough that somewhere hard work is way important than having a certain IQ.

At the end of the day, it is your hard work that pays off and obviously there is luck too but don't forget the harder you work, the luckier you get. And, please, do not think of yourself as an average student, tell yourself that you are good and smart and intelligent. I mean keep reassuring yourself that you will win this war if you stay on the battleground and keep fighting & hustling. Say positive things to yourself and keep practicing. 

 The following books might prove helpful in your preparation. 

First of all, for all subjects i.e Physics, Chemistry, Biology study the NCERT books cover to cover at least twice. For Biology read NCERT as many times as possible. For only the text material of Biology, you actually don't need to refer to any other book. NCERT is more than enough for biology. You might doubt this but trust me NCERT is really really enough for Biology. I can't stress more on this FACT.

For Physics, after completing NCERT complete all examples and questions from DC Pandey Objective book of physics by Arihant publishers. Revise the material as much as possible. If you complete DC Pandey and revise the material at least 3 times You'll be able to solve a lot of questions in the examinations. 

For Chemistry, after completing NCERT, for physical chemistry do MCQ type questions from Vibrant Academy's book of Narendra Awasthi. It is a book for JEE but for AIIMS you only need to do and complete the TYPE 1 questions from that book and that will be enough. You will find direct questions in NEET from this book. Trust me when I say this, for Physical Chemistry this book is more than enough for NEET. Follow it once, you won't regret it!

For inorganic chemistry, NCERT is enough. Read it as many times as possible. The same studying principle of Biology applies to Inorganic Chemistry as well.

For organic chemistry, solve Vibrant Academy's book by MS Chouhan. This book is specifically for NEET/AIIMS and will be enough to score high in the organic chemistry section.

For Biology, along with NCERT solve all questions from the book of MTG NCERT at your fingertips. Solve this book and revise it as many times as possible. 

LASTLY, your preparation would be incomplete if you don't solve previous years questions. For that refer to MTG chapter wise previous year question for all subjects i.e. Physics, Chemistry, and Biology.