Generally, duergar are more vicious than other dwarven subspecies, and like their nature, duergar names 5e often hint at this cruelty. After long enslavement by the Illithids (mind flayers ...
Using the Speak With Animals spell to convince the Rothe to work. Encouraging the Duergar to whip the Rothe harder. Using Force type spells, like Eldritch Blast, to slowly whittle away the stones.