1)
Why is Hashem called "Machaseh v'Oz"?
1.
Radak: He will shield us from the afflictions (Chevlei Mashi'ach - Sanhedrin 98b), "Chavi Chim'at Rega Ad Ya'avor Za'am" (Yeshayah 26:20).
2.
Malbim: This is one attribute of trusting in Hashem - He is a refuge and strength. Also, (b) He is a refuge not only at a time of Shalom, rather, also at a time of affliction. (c) One need not request much and wait for His help. He is very available and close to those who call to Him.
2)
Why does it say "Me'od"?
1.
Radak: It will be a great help, to save a small nation from many nations.