"Weak demons"??

Here's what I have to say. Without temptresses, demons will suffer such losses against archers and spellcasters that it will be worth restarting the game. So, I always take them. Even if it's against Sylvan archers – someone will take them out first, either the hero or the temptresses (and I've had that happen). If it's the hero, then it's a Dispel; if it's the temptresses, then we Charm them. So, losses can generally be avoided, even without using powerful units, if you do everything right.