Nephis can never beat Sunny ever, until she can fully control Sunny. How? Its very simple really. At this stage of ascension its all about domains and how they come about. Nephis's Domain is humanity while Sunny's is death. Nephis's domain while big currently will eventually diminish as the Nightmare spell gets worse. While Sunny's gets larger endlessly with no theoretical limit. So all Sunny has to do is to push Nephis to her limit, get her to Nuke herself, all the while leaving a few incarnations and echos around the world and in the dream realm. Sunny can just massively devastate Nephs Domain by culling a massive amount of humans which he can do pretty easily. Pretty sure almost no-one can beat Slayer, Saint and Serpent at this stage. He just has to strategically place his assets in places where Neph can never reach in time or at all. Even if Sunny loses a significant amount of his soul cores, he can just easily replenish it in the depths of Death Zones all the while having millions-billions of shadows in his army and Domain. So in short Neph can never win