Even if Lex told Z to hurry up, it was basically impossible for Z to quickly finish up a fight Lex didn't even think he could win in the first place. That didn't mean that Lex would interrupt the fight before it was needed.
Simultaneously, Z's fight could not be disturbed. Although Z's music was lethal to the locusts, under the protection of their king, they could pretty much ignore its influence and march right up to him. In truth, if they didn't rely on their passive ability and actively resisted the music, they'd be able to survive.
But even in their mindless form they were too difficult a foe for most, let alone if they gained a measure of intelligence.
Lex simply watched from the rampart as an ever increasingly nervous Kaemon stood by the sides. He was nervous not because he feared the locust king. Rather, he did not want the Midnight tavern to fall before he and his men returned to top form.
