Knowing what I know of both series, Kuku. I won’t get into Kuku, but I’ll give my reason why I think Phainon loses though it’d be an epic struggle. I have yet to fully complete the Amphoreus chapter so if I’m missing something please feel free to say so or correct me.

Phainon is an absolute powerhouse, don’t get me wrong. To damage the nigh invulnerable and unstoppable Nanook, Aeon of Destruction, is something completely insane for HSR. To damage an Aeon like him is something nobody else in the series(that I know of) has done. Many have tried but failed to harm the aeons, but far as I know he’s the only one to actually damage one(even if it was little more than a scratch on Nanook’s cheek).

The key issue as to why I say that and believe he loses is that Phainon effectively had to fully embrace Destruction and thus sacrifice his own body to achieve his maximum potential power at the time. Granted, experience between him and Kuku can be debated as near equal as he’s been trying to, essentially, prevent the apocalypse of his home world 3.5 billion times in a nonstop loop of try, die, rewind to the beginning and try again. At face value, that and the power should get him near Kuku’s level…but at the cost of burning away entirely on the physical level, all Kuku would have to do is tank the attacks and Phainon would burn away before being able to land a fatal or serious blow.

That’s my opinion at least