Well, here is what not to do to get an NOC from your employer

DO NOT FIGHT WITH HIM
DO NOT ARGUE WITH HIM
IF HE SAYS YOU WILL NOT BE GIVEN NOC, DON'T GET ANGRY

Unfortunately, NOC is completely the prerogative of your sponsor, there is nothing anyone can do about it. If you want to continue working in Qatar, there is no other way than to maintain a good relation with your sponsor. Otherwise, believe me, they can make life hell for you.

Good Luck