You can file as an individual or as an group. Since you have paid the money and builder is yet to complete the building you can file the following cases
a) criminal complaint against the builder (cheating case). he will be arrested and sent to jail, but a criminal complaint will result only in conviction(jail for builder). Your construction will get further delayed.
b) Civil Suit under Specific Relief Act: You can ask the builder to complete the apartments with in the specific time (new time lines to be decided by you) failing which you can ask the courts to permit the flat owners to organise amongst themselves and takeover the remaining works at the cost of existing builder.
c) If the flat owners take over the remaining works, these works can be funded from unpaid monies from flat owners and / or from attachment of properties of the existing builder.
d) Consumer court: You can file a compliant in consumer court under Sec 11 and 12. The consumer courts are very aggressive these days more so after passage of Real estate bill. Even builders like parsavanath, amarapalli, Shobha , Unitech, DLF, Mantri have been awarded hefty penalties and costs for delay in construction. the courts are awarding costs (interests upto 18% per annum, alternative accomodation costs, damages etc which are equaling 20-30% of monies invested by a buyer.
Option of approaching civil court and consumer court are available to you . You will need to decide. Once we know your full case history, we can evaluate both the options(in terms of benefits available to you) and then you can decide.
hope this helps.