What is "Ma'oz la'Dal"?
Radak: A protection for the poor, i.e. Yisrael. Malbim - it mentions also Evyon, who is poorer than Dal.
What is "Machseh mi'Zerem Tzel me'Chorev"?
Rashi: It is a tent covering to shield from Zerem (a flood) and shade from scorching sun. The Zerem is the spirit of tyrants. Their Ru'ach is like a flood that fells a wall.
Radak: The afflictions and Galus are compared to a flood of water and scorching sun. Hashem protects that Yisrael not perish in Galus.
Malbim: An Oni has no protection. You protect him from opposites - aridity and moisture. This is a metaphor for saving him from all his afflictions.
Why does it say "Ru'ach Aritzim k'Zerem Kir"?
Radak: Tyrants' Ru'ach, i.e. their anger against Yisrael, is like a flood that fells a wall. If not for Hashem's mercy, we could not stand! We find that Ru'ach is anger - "Al Tevahel b'Ruchacha Lich'os" (Koheles 7:9), "Kol Rucho Yotzi Chesil" (Mishlei 29:11).
Malbim: Hashem was a Kir (wall) to protect against tyrants' Ru'ach, which is like a flood that destroys. The start of the verse ("Hayisa") applies also to "Kir".
