advice for OP: look at things from their point of view, or put yourself in their shoes.. why are they lashing out at you this way?
my conclusion would be that they are severely insecure and are wanting to bring you down to their pit of self loathing.
just laugh it off and don't let that person make you miserable as well.

and that is what i do to remain calm. in any situation i look at the direct opposite. and if the opposite is a good alternative i strive for it. if it is negative i count myself blessed to be where i am. that usually relaxes me.