Legal remedy for delay in possession of flat purchased by builder

I have purchased a flat in Sidhdhivinayak Garima, Taloja, Panvel, Navi Mumbai on June 2017. The builder promised me to handover the flat until December 2017. But even after March 2018, construction of the society has not completed. When I ask about the possession builder says in a month or two he'll handover but in his RERA certificate, the postponed date is mentioned as DEC2018. It is a year delay from the original one. I am losing my rentals because of this delay. Can I file a case against the builder for this delay. Unfortunately, the possession date is nowhere mentioned in the sale deed. If I complain before RERA for the said builder, what documents do i need to submit? Will complaining to RERAQ only will do the job or I need to consult with a lawyer. Please suggest