How to get experience certificate from previous employer

Due to certain conditions, I was not able to serve the full notice period with my previous employer as I was overloaded with too much work and endured a kind of torture. After months of negotiation, I was able to get my original educational documents from them. Now they are not providing the experience certificate citing that I did not serve my full notice period. Adding to that I did inform them I need to be relieved early and I also asked them regarding buyout for which they declined. Am I entitled to get my service certificate? If so what should I do?