From what did he build the Mizbe'ach?
Malbim: It was from the rocks of the destroyed Mizbe'ach of Ba'al.
What is "Rosh ha'Ma'oz"?
Rashi: It is the top of the rock (mentioned in verse 20).
Malbim: [Build the second Mizbe'ach] on top of the first Mizbe'ach. It is called Ma'oz, for the first must be Oz (strong enough) to build another on top of it. Refer to 6:28:3:1.
What is "ba'Ma'arachah"?
Rashi: It is in a straight place at the top of the rock, that you can arrange there the stones of the Mizbe'ach.
Malbim: It is the woodpile on which the first bull was burned.
Why did Hashem command to use the wood of the Asheirah to burn the latter ox?
Malbim: Hashem told him to burn the animal prepared for idolatry, on a Mizbe'ach built from the rocks of the altar for idolatry, using the wood of the Asheirah, to show that He will redeem them from Midyan, and Gid'on will be Moser Nefesh to sanctify Hashem's name and show that idolatry is impotent.
