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

Dear user, where do you get these numbers from? 400 Imps by the endgame is the absolute minimum! And draining ALL the mana from a Warlock is usually not a problem. Of course, if your hero has a magic resistance skill, it might not be possible to drain all of it; then, the summoned Imps will suck up the rest... Of course, if the Warlock managed to find good artifacts, like a hat with +6 to Knowledge, or a Fountain that doubles mana, then yes, something will remain... but still, it will have to be used sparingly, which is a disadvantage.