Builder delaying the possession and not giving anything in written

Hi Team, I bought a house in Mar 2016 with assurance from the builder that the house will be ready within six months or maximum Jan 2017. During the booking, he took approx 30% amount and then later 40% additional as per the work status. He kept saying that I will give you everything in written but after taking the money he is not in a mood to give us anything in return what we agreed during the booking. I had disbursed all the money via check and loan so I have the payment proof. The queries are; 1) He committed to giving possession within six months from booking 2) I booked house in 2016 and was anticipating to get in the same year or maximum by Jan 2017 3) After Jan 2017, the taxation got changed and GST got introduced. Now, Although the project is getting delayed from the builder side and additionally, he is charging me GST where I suppose to pay service tax if he would have given on time. 4) He is not giving anything in writing but I kept video recording every meeting we have done so far, and he has confessed or admitted many times that he missed the timeline. 5) When I ask for compensation, He asks me to go for a case or legal action on him I just want to check, how should I proceed ahead. Is it worth full to go in RERA? and will RERA consider the evidence what I have in terms of recording since he is not giving anything in writing?