I recommend you writing letter to the MD for the issue of Relieving-cum-Experience Certificate. Mark copy of your letter to HR. Inform telephonically to HR about the request letter that you have sent to MD.
Wait for a week or so for the reply from MD. If there is no communication from the other side then approach Labour Officer (LO) of your area. Though LO's handle cases related to labour, yours is too small issue and possibly he will be receptive to your requirement.
Please note that LO is different from Labour Lawyer. Former is government authority and latter is legal professional who works in personal capacity. Generally employers do not refuse the requirements that come from labour authorities. Nevertheless, you need to be cautious on one more count. In the background check (BGC) your manager should not put a spanner. For this, tell LO about BGC also. Obtain agreement from MD that they will not create any problem in BGC. While at labour office, talk in local language. If you do not know the local language, then take someone with you whose mother tongue of that region.
All the best!
31st July 2017 From India, Bangalore