Why does the verse put together salvation of Amecha and Meshichecha?
Radak: Just like You went out to save Amecha when they entered the land of the seven nations, so You will go out and save them and bring them out of Galus and return them to their land. Even though it says Yatzata, Machatzta, Nakavta (14), past tense used in place of future; this is common in Nevu'ah. Even though Nevu'ah is for the future, it is as if it already was fulfilled.
Malbim: At the time of Ge'ulah, You will go out to save Your nation and Mashi'ach. Then Melech ha'Mashi'ach will be revealed, and he will fight Hashem's wars; Hashem will save him.
Who is "Meshichecha"?
Rashi: They are Sha'ul and David.
Radak: Mashi'ach ben David.
What do we learn from "[Machatzta] Rosh mi'Beis Rasha"?
Radak: It is the head of the evil army - Gog.
Malbim: If You will lightly strike the head of Beis Rasha, it will arouse and uproot the foundation of his body - the skull and brain, until the neck.
What is the meaning of "[Aros] Yesod"?
Rashi: The walls of their enemies.
Radak: Aros is Makor (infinitive). You will expose them until their necks. You will crush their head (Melech Magog) and reveal their foundation until their necks - the army officers with him. Also "Aru Aru Ad ha'Yesod Bah" (Tehilim 137:7) is revealing. The Yesod is the foundation of the building and what makes it endure. Therefore, it is a Mashal for the great people of the camp.
What is the neck?
Rashi: The height of the walls and towers.
Radak: The army officers with [the king]; the neck is close to the head.
What is the significance of "Selah"?
Malbim: It ends the second part of the Tefilah.
