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

It’s easier for Demons to play against mages because all the conditions favor them (leave them without mana, Marbas has resistance, and Alastor steals mana from mage archers). Plus, in response, creating a lot of Darkness units is the most reliable solution. The Warlock’s units are stronger in general, and opening a gate is decisive. If the Dwarves have Absolute Protection, the chances are roughly equal, but it’s different with the Elves; they will win without any problems.