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

If the opponent is a newbie, you can win without much trouble with any faction.
Although luck can influence the outcome of the battle (for example, morale and luck triggering, as well as special abilities of creatures). I played against a friend (Inferno vs. Inferno). My nightmares took out all his devils in the first turn.

It depends on how lucky you are with development!