Duryodhana did not change even though Lord Krishna showed cosmic vision (Vishwavarupam). Therefore, there is no use of arguing with such rigid people. He may say that the science will develop some technique in future so that the human body may withstand such huge power to lift the mountain! This means that the fellow will not accept the genuine miracle at any cost. You should not entertain him any more with any further discussion. Salute him and leave him with folded hands saying that he is stronger than the king of this country.
The king, having extreme powers, also may accept the logic in your discussion, but such fellow will never accept you and hence he is stronger than the king. Such fellow deserves highest regards, being more powerful than the king and hence you should salute him before you leave him.
Universal Spirituality for World Peace