What should I do if I am in the last month of the CAT exam and still not scoring even 50 marks and want to get IIM Ahmedabad?

Never treat your mock marks too seriously. Mock scores fluctuate for many applicants. The key is to not worry in the weeks before the test. Take as many mocks as you can in the remaining time to improve your plan, but don't overdo it. 

During tests, you should focus on answering the easy questions and skipping the hard ones. Analyze your strengths and weaknesses and work on a plan that will help you get the best score possible. Be clear with your strategy as you can not do much in the last weeks other than make a strategy to solve the paper in the correct manner.

Also, don't try to learn too much in the days before the test. Review the basics, look over your previous practice tests, learn from your mistakes, and take a break before the exam. You can get into the best colleges and even IIMA.