Adam Clarke on 1 Kings 20:23

Their gods are gods of the hills - It is very likely that the small Israelitish army availed itself of the heights and uneven ground, that they might fight with greater advantage against the Syrian cavalry, for Ben-hadad came up against Samaria with horses and chariots, 1Kgs 20:1. These therefore must be soon thrown into confusion when charging in such circumstances; indeed, the chariots must be nearly useless.

Let us fight against them in the plain - There our horses and chariots will all be able to bear on the enemy, and there their gods, whose influence is confined to the hills, will not be able to help them. It was a general belief in the heathen world that each district had its tutelary and protecting deity, who could do nothing out of his own sphere.