1)
Who says "me'Es Hashem Haysah Zos"?
1.
Rashi (27, citing Pesachim 119a): David's brothers said it when Shmuel anointed David.
2.
Rashi: Everyone will say so [in the future].
3.
Radak: The kings of the land will be astounded, and say that the salvation of this nation was only from Hashem.
2)
What is "wondrous in our eyes"?
1.
Radak: The kings of the land say so about the salvation of Yisrael, via Hashem.
2.
Malbim: [The salvation from Machaneh Ashur] was a miracle.