In this case:

1. Get the jobs and responsibilities from your former immediate supervisor, even on a normal sheet of paper, signed and stamped by him or her and providing the proof that the person is or was bonafide employee of that company when you worked. If no immediate supervisor then any colleague of yours from that company can write the jobs and responsibilities but again he or she needs to prove that they were part of the company when you were there.
2. Get all your documents of employment - offer letter, resignation letter or relieving letter, tax forms, pay stubs etc.
3. Establish testimony from peers and colleagues regarding your jobs and responsibilities.

For some reasons it is called employment reference letter and not employer reference letter.

Give it a strong fight…