Big realestate client not paying invoices

We are a small company and have done work for a realestate company which is among the top 10 land developers in India. Invoices are not being cleared by them for the last 10 months. The payment terms mentioned in purchase order was 60 days. The pending bills are more than 15 lakh rupees and the resultant cash crunch is killing our business. The experts advice on the legal course we must take will be really appreciated as we are no match to the customers legal help.