Against which factions is it easier for demons to fight, and against which is it more difficult?

It's easy to win against anyone if your opponent is a newbie. But if you know all the nuances of each faction, then it's Orcs, Fortress, and Sylvan. And even a Demon versus a Demon isn't easy (I tested this with a friend: we gathered roughly equal armies, and my Nightmares wiped out all his Devils on the very first turn).