Even if those monsters were completely unbothered by injuries, so what?
No matter how agile those monsters were, they were inevitably hit by the Necromancer's area of effect attack. As such, without any accidents, the Necromancer's attacks landed on the monsters most of the time.
Although those monsters still retained enough mobility to dodge some of the Necromancer's attacks to an extent, the situation was such that even if many of the attacks missed their targets, as long as a small number of attacks hit them, it was enough to kill those monsters.
Considering the monsters' conditions - even a single scratch by the Necromancer's attack was enough to kill them. The Undead Domain stopped the monsters from recovering HP. While the King-level monsters who were of crucial combat power were fine, the ordinary monsters couldn't get any healing from Zhou Tian's team. After enduring the attack with the last dregs of their health, their fate was entirely predictable.