Why does it say "Yidkeh Yasho'ach"?
Rashi: So is an ambusher's way - he lowers himself so he will not be recognized (seen).
Radak: This is another way that the Rasha catches Aniyim. He and his clan make themselves appear crushed and bent, so people will not guard themselves from them. They go together with them on the road. When they are far from settled areas, they show their strength and seize them.
Why does it say "v'Nafal ba'Atzumav Cheil Ka'im"?
Rashi #1: Into his enclosure fell a legion of the broken (Aniyim). Mesores lists Chalka'im among 15 words that are written like one word, and pronounced like two, like "va'Tomer Leah Ba Gad" (Bereishis 30:11), "Esh Das Lamo" (Devarim 33:2), "Mah Lachem Tedak'u Ami" (Yeshayah 3:15). Ka'im is like "v'Nich'ah 1 Levav l'Moses" (below, 109:16). Atzumav 2 is like "v'Otzem Einav" (Yeshayah 33:15).
Rashi #2: The weak fell [into the Rasha's grasp] via his Giborim. R. Simon said, this Rasha cast in his army only Giborim like himself - "ul'Guvrin Giborei Chayil Di v'Chayle" (Daniel 3:20). Menachem explained that Chalka'im is poor, like Chelech; the Chaf is from the root.
Radak: Into his limbs fell a legion of the crushed and weak. Atzumav is an adjective for his [strong] limbs - his arms and legs.
Malbim: Also when he is crushed and bent, in his old age, the weak fall prey to him. He damages in all three eras of his life - youth, standing [in his honor] and descent.
Rashi: Do not say that the Nun is from the root. It says "Hach'os Lev Tzadik" (Yechezkel 13:22). Rather, the Nun is like in "Nimvezah" (Shmuel I, 15:9) and "Nekalah" (Yirmeyah 6:14).
Rashi learns also from "Hagishu Atzumoseichem" (Yeshayah 41:21). I do not understand this. All Meforshim there, including Rashi and Targum Yonasan, say that it refers to claims! (PF)
