Remedy in case of non payment of rent by constructor

I had purchased a studio apartment in raigarh, MH on the promise that it can be handed over back to them with a monthly rent back of 7500 rs and 5%annual increment. however their payment has been irregular to the extent that about 3lakhs is pending from first 5 years. also in past two years they hv reduced rent to 5000 a month that too not paid regularly. however they hv always promised that they will pay but hvnt done so. lately they hv been very rude and threatening to finish the contract. what should i do. i hv shown my concern to them many times through emails.