Although Pan Shi had some knowledge of the Main Plane, ultimately, because he did not return to the Main Plane, his understanding of it was only through some special tactics. As for the information regarding the Human Emperor, even the Deity could not have prior knowledge of it; even with Pan Shi's abilities being strong, it was impossible for him to know the specific details about the Human Emperor.
Given the situation at the time, even if Pan Shi was incredibly strong, because he did not understand the Human Emperor, let alone how to break the Human Emperor's immortal body, Pan Shi couldn't find a direction to attack when he wanted to deal with Zhou Tian.
If he could not break the Human Emperor's immortal body, then Zhou Tian would be an undying existence. In this situation, no matter how powerful Pan Shi was, what was the point?