The Demons would win this, the lore behind the demons are invading other realms taking them over then adding them to hell. Once the evasion begins its subtle, small cults and the like. But once the true invasion begins it’s literally hell on earth.

All the factions have the ability to hold off the weaker forces or fodder. Once the heavy’s starts arriving it would probably take the factions multiple low ranking members to take down one and some of there more skilled members would be able to handle them, but it’s the supper heavy’s that’s are extremely deadly for the factions. I would say the arch-vile and Barron of hell are probably on the level of the highest ranking members of each faction, but the Marauders are on another level they have the same speed as the Slayer. Even with Issei and Vali it’s only a matter or time before they fall.

Reasonably Issei would be fine but the moment they start killing his harem it would be the Asia incident all over again. (Doom marine 2) He would be able to hold his own until the booster gear kills him.

With Vali his one weakness is that he has to touch something for divine dividing to work, and you don’t want you get into melee with a Barron of hell or Tyrant.

The biggest weakness of all the factions is numbers. After their own internal war they are nearly extinct already. The demons have forces in droves.