Delay in possession, compensation and GST

Hi, I purchase flat in Mumbai area in May 2015. The agreement says Mar 2015 as possession date with 6 month buffer period making it Sep 2015. The flat it still not ready as of today. The builder always give false commitment to each buyer. The project update in email is not in sync with that of current project work. Builder is not answering our calls. 3 months back he received entire CC and informed that Jun end will be soft possession and in 2/3 months he will get OC. The ground reality is different. it will take another yr to complete this project. Builder is not responding to the emails or calls. He has made structural changes without updating flat owners. The podium was supposed to be for car parking but he built 3BHK and sold it one party. We are paying pre EMI, monthlly rent. Few people have book flata in Mar 2013. We have paid 95% of the total amount and now with new GST coming up he is asking to make rest of 5% amount to avoid GST. What options we have? How we can deal in this case? What compensation option is available? How we can pressurize the builder? What all things needs to be done.Recently few of us filled a complaint in police station but no update. What can we do step by step? Can we form Society? We are asking RERA registration no. which builder is not proving us Not sure if he has registered. Its because of delay from builder we may have to pay additional GST, If we file case in consumer court how long it will take? Is it good to go with RERA or consumer court? what is option in this case. Please guide.