I know it is very difficult to deal with people who try to pull you down. I have faced it, and to be honest, here you need to use the tactic tit for tat...being nice and polite won't help here.
Initially, when there were such people around me who used to do that I would stay mum but later on fight it within my head, eventually get stressed being around such people. Such people need to be given back with the same amount of criticism. You may not like it at the beginning, but slowly it would feel better. At least it is better off rather than cursing ourselves as to why I couldn't give a strong reply to what s/he said.
You may not agree with what I said, but this is my way of dealing with such people. I am cultivating this bit in me to give it back at that moment rather than bottling it up until i burst!
Mousumi, If this is your way of solving a problem and comfortable with the way, why not? After all, the problem is yours. Go ahead. Don't wait for others validation.