The strength of NPC Demigods was extremely formidable. If they could really be killed off by a volley from a Centaur army, then Zhou Tian would not have to treat those Demigods as formidable enemies. Having witnessed the strength of those NPC Demigods, Zhou Tian had predicted early on that it would not be possible to kill them off with his arrangements.
Zhou Tian never believed that his tactics could defeat those NPC Demigods. His true objective was merely to use that tactic to tie up some of the Demigods' energy, so that he could be under less pressure when he took action himself. Then, with his other preparations, he was at least seventy to eighty percent confident that he could repel those NPC Demigods.