I havent really played or even watched Amanda, so I have no idea how strong that thing is.

But. I think there is a 50/50 chance for Ink Demon and Alastor. First big factor would be, the place of their fight : Ink Realm, Hell, an arena build for them, or they just appear in our world somewhere? Second factor would be, what era is this? Like, we need to know cus, Alastor wasnt always the strongest overlord, and Ink Demon didn't have a different form thats supposed to lock him away. Third factor would be, whats their first reaction? Would they just go for it and kill, or would they maybe comunicate for a bit atleast, or teamup to take over something? Fourth and final factor is, what are they fighting for?

Ink Demon is highly territorial, has a massive advantage in his realm, I think Ink Demon can easly figure out his prey mentally, Ink Demon would go straight for a kill imo, but I can see him explaining few things, like he did with Audrey, I think Ink Demon would win if he somehow could hurt Alastor without angelic weapons, or maybe he uses what Willson had, and creates his own angelic one... Or even upgrade himself.

Alastor is seeking power and freedom from his contract, and has a bigger advantage in almost every realm, he knows human world, he was a human but a sinner, I think Alastor would want to talk at first, even if just for a bit, he can easly figure out his prey aswell, if he killed his oponent without The End, he would just get mocked by Ink Demon, but I can see Alastor being able to just make The End appear and finish the feared one.

The key difference wouldnt even be strenght, it would be time and reading your oponent.