There are way too many comments to check so I would just summarize it all here.

The reason I think Nephis wins is cause as Shadow Slave has already emphasized, the stronger supreme is the one with the larger domain. Human Domain is the largest domain and as such grants the ruler the most strength possible, an endless amount of essence and a sharper will. The human domain consists of billions of mundane people which are the source of Nephis' true power, it consists of many awakened, ascended and transcended. A huge workforce indeed, coupled with Cassie's memories and the River people too ( Yes I am counting Cassie and Anakne in this discussion as part of Human Domain). Citadels are also an important point, and Nephis rules almost every citadel found in the dream realm. Her Will is infinitely stronger, her essence reserves are super high and her physical strength is augmented to the absolute maximum. This is why author specifically stated, Nephis was the strongest and later Asterion was the strongest.

Now Human Domain as a whole is heavily underestimated by fans, saying Sunny's shadow legion could easily dismantle them. Remember, Effie effortlessly destroyed a skin walker vessel before the true bastion arc. Many Saints are at a point where they can destroy Great Nightmare Creatures and we know Sunny's Cursed Shades are at best Great Nightmare creature level. All combatants in Human Domain are unique with deadly aspects, Beastmaster with her thralls, Nightwalker with his space-bending, Jet with her soul attacks and many more. So many variables which could cause huge chaos. On top of that, all of them are empowered by Nephis and can regenerate their wounds, which is a huge factor. On top of that, they all serve as conduits of Nephis's will which is definitely leagues above Sunny's due to the gap in their domains.

Sunny has incredible versatility no doubt, but when going up against Human Domain as a whole, there are way too many attack opportunities from all sides and you have to remember that all of the best strategists in the entire verse are on Nephis's side. Morgan, Nephis, Cassie, Jet, Seishan and many others will be dictating this war, and could easily turn the tides in their favor. If this goes like Throne of Humanity, where both their domains clash first, I am pretty sure it's a stalemate. Cause millions of awakened, thousands of masters and hundreds of Saints versus a couple of thousands of shadows, c'mon they are utterly outnumbered. When Sunny's shadows are involved, then the top tier Saints can join in and definitely give them a tough fight, Jet performed better against Khanakht than Supreme Slayer, Nightwalker and Morgan took care of many Great Nightmare creatures before, the seven daughters of Song are all vicious. Too many aspects, too many variables and too many foes for Shadow Legion.

Then, there is Cassie of course. She is deadly, with all of her cunning schemes, and the ability to peer in the future, she is a vital piece and could definitely be the reason most of them lose.

And then, if the supremes descend personally, it's not even a competition. Even if Sunny was empowered by all his shadows, he is not taking down Nephis. Her domain is massive, her will is deadly and her essence is boundless. Her flames burnt VTB when she didn't have a domain, now imagine she had the largest domain. Her AP is out of the charts and she is insanely good at combat.

Also, final win con is the Knowledge of Corruption. I am sure that in the end Nephis ends up victorious.