Amenities not completed by builder and also this project not comes under RERA projects. How to deal?

Dear Sir/Madam, This is one of the projects in Bangalore which was started in 2013. Builder and his sales team promised to deliver occupancy certificate by 2015 december. We have received occupancy certificate copy between 2017-18. Now somany amenities not completed yet and it is been 7 years and from 2016 November builder left the premises and managing few owners and 2 staff members to just to know what's happening in apartment. If he feels some one is going in any complaint direction he will start a small work and will finish that in very unusual time limit.Like that from last 4 years he hardly completed things. If I say 10 pending amenities he completed so far 4 amenities in last 4 years. If he go like this it will take another 10 to 15 years time to complete all and by that time apartment appreciation will completely go down. this is around 200+ flats apartment.With no other choice we as owners formed as an association in 2018 and somehow managing apartment. We did not receive single rupee from builder as corpus or any form. On top of that he is asking money by naming it as dues. May be few flats may have some dues to builder and now at present they are also hesitating to pay as work is not progressing. He also has not handed over any documents to association and not allocated car parking as of today. To prove that he has not absconded he used to manage with 6 ,7 owners and they will manipulate things in society with builder direction. Finally our query is we are not able to gather people to file a case or complaint. We registered our association under 1960 act. Could any legal expertise team suggest what is the best solution or approach for our case to make builder to complete amenities in speed manner.We already tried sending registered posts to all possible channels and all posts are returned with unavailability of address. Please advice !! Krishna.