What does "Yadin ba'Goyim Malei Geviyos" refer to?
Rashi #1: He judged dead bodies of the Egyptians at the sea. "Malei" is a gathering (here, of dead bodies). This is like "Kar'u Acharecha Malei" (Yirmeyah 12:6).
Rashi #2: In the days of your descendant Chizkiyah, He will judge the multitudes of Sancheriv's army, a gathering of dead bodies.
Radak: Hashem will do judgment on the Goyim until the place of the war will be full of dead bodies.
Malbim: Hashem will judge the Goyim of Amon, to smash the bodies, to pass on them iron threshing tools.
What is the meaning of "Machatz Rosh Al Eretz Rabah"?
Rashi #1: This is like the Nevu'ah "Machatzta Rosh mi'Beis Rasha" (Chabakuk 3:13) - Pharaoh, who was the head and general of a great land, and esteemed above all lands, like it says "Moshel Amim va'Yfatechehu" (105:20) - all nations were under Egypt's rule.
Rashi #2: He will strike Sancheriv, the head of Ninveh and Ashur, which is a great land.
Radak #1: [David] struck many leaders on the ground. "Rosh" is a Klal (collective noun), and also Rabah, like "b'Sod Kedoshim Rabah" (89:8).
Radak #2: Rabah is [the capitol of Amon] - "Rabas Bnei Amon" (Yirmeyah 49:2). This verse refers to that war.
Malbim: [He will judge them] after He smashed the head of Eretz Rabah, i.e. the capitol, at the head of the country, and the king, who is the head of the nation. The verse depicts that the bodies have no head, i.e. king. Hashem will judge the city full of headless bodies.
