When the nature is upright, why fear monster or demon? However high the mountain, it cannot make metal beget earth.

不得 here is the modal one, standing in front of a verb rather than after one, so 得 keeps its full dé and means cannot. The five phases generate in a fixed order and earth begets metal, never the reverse, so the line says the mountain cannot overturn the order of things.

That one's pestle parries like a python coming out of a pool; this one's rake arrives like a dragon leaving the shallows.

那個 and 這個 keep the camera moving between them, and the animals are graded: a python for the fiend, a dragon for Bajie. Both come out of water, and only one of them is a god.

Shouted cries and bellowing shake the hills and rivers; the roaring and its manly force startle the underworld.

喝 is hè, to shout — not the hē of drinking. In 咤吒 both characters are the same word for bellowing and both read alike, so 吒 is zhà here and not the zhā of the god Nezha's name. 地府 is the bureaucracy of the dead, and reaching it is the conventional measure of how loud a fight has got.

Two heroes, each showing what he can do, throwing their bodies in and staking their powers on it.

賭 is to gamble, and the last word of the poem makes the fight a wager rather than a contest. The rhyme has closed seven for seven on 杵, 雨, 主, 土, 浦, 府 and this one. And note that both of them are called heroes.