1. Hope you have adequate documents/papers to prove your allegations,
2. If not, collect the same like brochure, assurance of the builder for early completion, Bank's refusal to pay instalment for non completion of phase work, builder's charging interest for the said delay etc.,
3. After collecting all the evidence, file a complaint case before the local District Consumer Dispute Redressal Forum alleging deficiency in service and unfair business practice followed by the builder claiming immediate possession & registration of the flat and rent for the delayed period, interest, damage & litigation cost,
4. Your other flat owners also can join you,
5. It is a very good case to win.