Power creep is a bitch. Sure you wanna go with the epic shit where you punch out satan, but after that, what next? Are you gonna go full Gurren Lagann an eventually have people using Galaxies as Throwing Stars?
Who's the story for? You? Then go with your bad self. If the story is to try and get a loyal following, start small. It's always easier o make the MC more powerful will time. Its hard to go the other way.
Fr example, MC in one story is an angel with magical power to do... anything. However, he can only do anything very weakly. So he can teleport like 10 feet. He can shoot bolts of flame that feel like getting hit with someone using a lighter and a can of Right Guard. Impressive blast of flame, but of little consequence.
But he can do weaknesses very well.
So, in he rushes, glowing with holy light, keeping the weak undead at bay so the rest of his team can gang up on the main undead badguy.
And the bad guy hits him with a darkness field. Completely shuts him down and while he's flailing about, mops up the rest of his team.
Now The MC is completely FUCKED and not only has to figure out how to save his own ass, but the rest of the team.
You can relate to that, I think.
Now, you want me to relate to a mutherfuckin servant of GOD who is most likely experiencing life in a way I cannot comprehend, existing in dimensions I cannot perceive, who fought in battled that spanned eons against titanic forces enough that It caused this entity to suffer PTSD, to the point where he is now GENOCIDAL.
Do... you understand that term?
This is a being that will, from your own description, laser focus on the task of utter destruction of a RACE. A creature like that would be the MONSTER I would be fighting. He'd learn a demon was hiding in a village, and nuke the village without a second thought. Hell. Unlikely to find a first one.
Some woman got raped by a demon. Had a kid. Kid's fairly normal. Angel finds out, Murders the child. WITH A COMET.
What you have described is a remorseless, inhuman murder machine bent on the completely annihilation of an entire SPECIES. Is this the MC I'm supposed to relate to? Admire? Like?
I mean, as a character study, it might be interesting to get inside the mind of such an alien thing, but can you give me that lien mind set? I'm talking Blue Orange Morality here.
As for the demon... Okay? I mean, at least he would most likely have to pretend to be human and act human in order to worm his way into world domination. But uh... why?
I mean, why rule the world? 'Cause Demon? Demons just do that shit? All Demons? Some demons? Why are there demons? I mean, you are presenting me with an apocalyptic war between God and Demons. 'Kay... I need to know the cosmology of this to even take it seriously.
I'm getting around it by explaining the physics of the matter. Angels are made up of different matter than humans. Gods don't come down and fight because they are fighting other Beings of the same type.
Imagine you are fighting someone else in a sword fight. Below you are millions of Ants. On the other side are people, like you, with millions of ants below them. Now, a million ants can ruin someone's day, but here's the thing. How do you help the ants? If you just start tossing lightning bolts, are you gonna help the right ant? If you pause and CAREFULLY help the ants, the other guy is going to take advantage of your concentration and jump you.
Also they need faith energy to safely manifest or if they force it, there is backlash. Just talking to one ant might unleash a tornado if the power isn't handled carefully.
SO, I have Gods fighting ABOVE the MC and the mortal forces can kinda get help, but if the gods focus too much attention on one part of the fight, other parts of the battle will suffer.
So, why isn't god just smiting the demons and "winning"?
You want this scale, you need to know these answers, even if you never tell the reader, because if YOU don't know, it will leak into your story. You will make mistakes, the reader will noticed, and you'll get called out for it.
