Dear Naman,
You are a service provider who provides content writing services to your client. However, before commencement of your services, did you enter into a formal contract with the clients? If yes, then did you enumerate the process for providing the services, acceptance of the services and further how your invoices will be cleared? What evidence you have to prove that you had provided your services to the client? Whenever, any content was provided, did you assign any reference number? In your invoice, did you quote this reference number?
Your case falls under the purview of Indian Contract Act, 1872. For failure to pay your legitimate dues, you can file a suit, however, without formal contract and proof of your services, the case will become difficult. If formal contract does not exist, then also it can be proved that implied contract exists. However, you need to find a right lawyer who can do that. Secondly, what is the amount involved that also matters. As you are well aware, litigation in India moves at snails pace. Amidst this, how far it is advisable to take the course of litigation that remains to be seen.
By the way, my above suggestions are based on what you have written on the forum. In fact, any suggestion needs to be given after understanding viewpoints from both the sides.
Thanks,
Dinesh Divekar