1)
What is the meaning of "k'Esh Tiv'ar Ya'ar"?
1.
Radak #1: It is fire that burns b'Ya'ar.
2.
Radak #2: Tiv'ar is Po'el Yotzei (it acts on others; it burns the forest). Once fire burns a tree, it burns another tree [next to it]. So the enemy will be due to the panic that Hashem will send on them; a man will kill his colleague. Malbim - "va'Yhi ha'Am k'Ma'acholes Esh Ish El Achiv Lo Yachmelu" (Yeshayah 9:18).
2)
Why does it say "Telahet Harim"?
1.
Radak: The wind blows on the trees and vegetation on the mountains, and pushes the fire from tree to tree until all are burned.
2.
Malbim: The flame burns [what is on] the mountain. So Hashem's flame and Machaneh Yehoshafat [will consume the enemy].
