What is the meaning of "Yaharosu"?
Rashi: They were destroying.
What is the meaning of "Ad Hish'ir Avaneha ba'Kir Charases"?
Rashi: They took rocks from the wall until the rocks were on the Charsis (mud), i.e. no rocks remained.
Radak: Kir Charases is the name of a great city of Mo'av (Yeshayah 16:7). It is mentioned in every [episode] of the fall of Mo'av. Since it had a name, only it is mentioned, but not the other cities destroyed. They destroyed all the other cities, but not it, for it was fortified. In it, they left only the rocks of the wall, but destroyed everything outside and felled the trees.
Radak citing Targum Yonasan: A rock did not remain in the wall that they did not destroy.
Malbim: They destroyed all the other cities, except for the wall of Charases, for it was made of great rocks.
What is the meaning of "va'Yasobu ha'Kala'im va'Yakuha"?
Rashi: They struck what remained, i.e. walls that were not toppled, and toppled them.
Radak, Malbim: They struck the wall of Kir Charases (the only remaining city) until they destroyed it. Radak - they did not need Kala'im (projectiles) for the other cities, rather, they conquered them and afterwards destroyed them. Since this was a fortified city, they needed Kala'im. These were not slung by hand (slingshot), rather, big heavy rocks were catapulted against the wall until it broke.
