Builder not giving possesion

It is always proper to fight collectively against builder through consumer forum  as party in person as you individually can not match a Builder.

Indeed, you can file a case in the consumer forum (or the state commission if the payment made by you exceeds 20 lac, or the national commission in case the payment exceeds 1 cr.) Note that you first need to send a legal notice to your builder, claiming refund of the amounts paid along with interest and/or damages, and if he/she doesn't respond within the prescribed time, you can move immediately to the court.