1)
What is "She'ar"?
1.
Rashi: It is the remnant.
2.
Malbim: The enemy will intentionally leave over some Giborim [of Kedar] to fight its war.
2)
Why does it say "Keshes Giborei Bnei Kedar"?
1.
Rashi: They were archers, like their ancestor - "va'Yhi Roveh Kashas" (Bereishis 21:20).
3)
Why will they be diminished?
1.
Radak, Malbim: This is Hashem's decree, for they did evil to Yisrael. Radak - perhaps they came with Melech Ashur and harmed Yisrael when they took [the 10 tribes] captive, or Yisrael passed through their land, and they harmed Yisrael. This is why it says Elokei Yisrael!