Will Hashem put His hand on them only once?
Malbim: No - it will be time after time, like I explained in Yirmeyah Perek 47.
What are "Keresim"?
Rashi citing Targum Yonasan: A nation liable to be destroyed.
Rashi: Pelishtim are called so - "Hoy Yoshevei Chavel ha'Yam Goy Keresim" (Tzefanyah 2:5). No other nation is called so! It seems that there is a border in Pelishtim called Keresim. We find "Anachnu Pashatnu Negev ha'Keresi" (Shmuel I, 30:14).
Radak: They are a great family from the Pelishtim. Also "Pashatnu Negev ha'Keresi" and "Goy Keresim" refer to them.
What is "[She'eris] Chof ha'Yam"?
Rashi: It is the frontier, i.e. Pelishtim who dwell in the southwest corner of Eretz Yisrael, by the Mediterranean Sea.
Radak: (It is what remains on the shore.) Ako (which is on the shore) was destroyed with the Churban of Eretz Yisrael. Also Eretz Pelishtim has a part on the shore; it remained after Churban Eretz Yisrael. I will destroy it.
Abarvenel: Part of Eretz Pelishtim is on the shore. Ako, the end of their border 1 , was destroyed with Churban Eretz Yisrael, for Eretz Yisrael extends until the [Mediterranean] Sea. There remained from Eretz Pelishtim the part not on the shore; I will destroy it.
Ako is partially in Eretz Yisrael, and partially in Chutz la'Aretz (Yerushalmi Gitin 1:2). However, it is the northernmost part of Eretz Yisrael (Gitin 2a, 7b). The shortest path from Egypt to Eretz Yisrael is via Eretz Pelishtim (Shemos 13:17), i.e. Eretz Pelishtim is southeast of Eretz Yisrael! Abarvenel must hold that Pelishtim had land in the south and in the north, and Eretz Yisrael divided between them. (PF)