Builder's claim on non completion of flat & no Occupancy certificate

I have bought a flat,82% of total payment disbursed at the time of sale deed.My 10% of total amount is due on flooring completion and rest on flat completion. From Nov 2017 onwards builder has sent me demand letter two times stating that flat is completed. After my visit to flat there are lot of issues pending which includes few of flat specific and majority of amenities specific issues.I shared them a list of issues pending on 2nd Jan 2018.when I shared issues and cleared that I am not going to clear any amount. They sent me a legal notice stating that their flat was completed fully and i need to clear full money along with penal interest.later they agreed on mail that,release only flooring charges as per agreement. Occupancy certificate still not shown to me after many mails .moreover they are not taking care of any issues they are saying will be done you release amount related to flooring completion. What to do with this notice does it still consider valid?how to deal with it.