How should a student deal with personal criticisms from an assistant professor?
He may be competing with your supervisor. He may not like your general topic. He may not like you. Or, he really believes what he says. Or he wants to test you. You don't know.

You have to live with people who confuse what's going on with prejudice- or agenda-coloured "truth". Practically all successful researchers have encountered such put-downs. 

Schechtman (later Nobel winner) got a "Introduction to Crystallography" book put on his table when he first reported that he got a 5-fold symmetry, hence quasicrystals, in his experiment. Feynman got put-down big time initially by Oppenheimer and was only saved by Dyson's aggressive intervention.

You think your progress is good? Your supervisor does, too? That's enough for you to go on, then. This guy may be a big shot, but for you, it doesn't count. Big shots are usually right when they think something is cool/great. They can be awfully wrong when they think something is not good. Trust your instincts.