Mental Harassments By Builder

I have purchased a flat from builder in 2017. At the time of purchase, the builder has promised that they will provide assured rental for 2 years after possession of the flat. The possession of the flat was taken in Feb 2019 but assured rental from the builder has not started. I have following builder from last one year but they are not responding. Now I want to file a case against builder for mental harassment. I have already won a case against builder for delay in possession. Now I want to know how to make my case stronger and what are documents required by the court to implement a steep penalty on the builder.