1)
What is "b'Nihem"?
1.
Rashi: It is their Nehi (lamenting).
2)
What is "[ch'Tzor] k'Dumah"?
1.
Rashi: This is the name of the heads of Edom, like it says "Masa Dumah" (Yeshayah 21:11).
2.
Rashi citing Targum Yonasan: [No one is] like her.
3.
Radak, Malbim: Who was like Tzor in her level, and who is like her [now], Domeh (cut off) in the sea? This is like "Shachenah Dumah Nafshi" (Tehilim 94:17).