Alduin's Dragons could utterly destroy the Dremora but Dagon would win. If Akatosh, Alduin's Father, was only able to essentially kick him out yet not kill him, what makes you think Alduin has a better chance.
It would be a stalemate. Going by game mechanics, Alduin is invincible unless suffering from dragonrend....but that just means they possibly can't do anything to each other aside from giving one another nasty looks.