How should a new mum combat negative remarks made from the elders during the early days of breastfeeding when the milk supply has not kick in?

I would say, smile politely and excuse yourself to go into the room to rest (it is during your confinement period anyway). The more important thing is not let their comments get to you. They are just not well-informed and there is no need to get into any "discussion" on this with them (assuming that they are probably not open to learning more about breastfeeding).