I'm playing as Dirael. Summoning magic rules even against demons; the wasp swarm is a game-changer!
Generally, demons are a strong opponent. In battle, you need to solve the following problems:
1. Jaleb's Ballista.
2. Dark Magic.
3. Demonic Gates.
4. Imps!!! This point is often underestimated. Meanwhile, they drain mana, and if you don't kill them immediately, you'll have nothing left to cast with later.
5. Succubi with mind control.

Demons are exactly the kind of case where you need to break through as quickly as possible. Troops must reach them on the very first turn and deliver a crushing blow, after which one same-turn delays, summons, and the ballista will no longer save them. There is no chance in a long battle. From this, we conclude: the hero must be built for maximum damage plus ATB scale pushback, which is also necessary to maintain maximum damage.

Hence, the following build for Dirael:
1. Summoning magic with wasp swarm — for ATB scale pushback. An Arrow Rain charged with a wasp swarm can push back three sworn enemies at once. When choosing sworn enemies, I base it on the need to deal maximum damage. Consequently, it is important not to let the enemy take our units out of the game. Succubi can do this via mind control and horses via fear. Therefore, they should be set as sworn first, and the last slot, obviously, for Archdevils because if they wipe everyone out, there will be no one left to attack.
Further progression in Summoning magic beyond what's essential for the battle with the enemy hero doesn't matter. In the fight against a demon, its task is limited to the wasp swarm and a fast start on the map thanks to that same swarm and fire elementals. Everything else is secondary. Therefore, ideally, it's better not to level Summoning magic beyond Advanced and Fire Warriors.

2. Logistics and Keen Mind. It is important that demonesses and nightmares move as late as possible. Thanks to Keen Mind, the battle will start with us either casting a general haste to get the chance to strike with unicorns and bowmasters, which can push back a couple of stacks, or we'll use the wasp swarm to push back the demonesses and nightmares ourselves. It's better not to level Logistics beyond Advanced.

3. Leadership with Speed Aura and Forest Leader. Speed Aura ensures a fast start to the battle, and Forest Leader provides Retribution in the attack branch. Additionally, 10 dancers on the second week or 20 on the third can help with the start. Combined with fire elementals, this is good fodder for breaking through. So, we already have +1 speed from the aura. All that's left is to find boots and a fast start to the battle is guaranteed.

4. Attack with Retribution and Mass Haste. In total, with good Leadership, our troops will have +40% damage and +30% initiative. Plus +1 to max and min damage. This is exactly the skill that ensures maximum damage. There's no point in taking Tactics, as the probability that demons take it too is very high, so it won't help us reach them quickly, whereas haste and hyper-damage are just right. In general, the combination of Leadership and Attack is by far the most important part of our build, requiring 12(!) skill points. Immediately after getting fire warriors, you need to level one of these skills, ideally starting with Leadership and Speed Aura, as this strengthens the strategy of clearing neutrals using fleeing dryads. By level 7, we can have both fast dryads and fire warriors, and calmly clear even the same-speed melee units.

So, all the above skills can be had by level 20, considering that Arrow Rain had to be taken for Retribution. Add an Enchanted Arrow to this and you get a powerful level 22 hero. At the same time, we chose skills with a reasonably high drop probability, so obtaining such a hero is quite feasible and shouldn't cause major problems. Further progression is not as important. The main thing is to grab a couple of anti-magic scrolls or a Tome of Light so that mind control doesn't ruin your life too much, or similarly, one same-turn delay scrolls and a Tome of Darkness.

5. One skill slot remains. If we are lucky, either Dark Magic or War Machines will drop. In the first case, you can set two Seals of Darkness and get mass slow while having mass haste. We can now answer mind control with mind control, and the demon is NOT scary AT ALL! The second option is nice because with an Enchanted Arrow, we can cast the swarm another two times per turn, and combined with a rain of charged arrows — 5 times! In this case, it's better to shoot at imps and kill them faster so they don't drain mana. They likely won't be able to act more than once. If you have machines, you can even hit them with archers if the opportunity arises because mana has become more important. There is one more plus from machines: it's easier to take out the enemy ballista. Only then will you need to charge the arrow not with a swarm, but with Chaos magic for a while. If the enemy happens to have strong defense and a powerful ballista, this would be quite reasonable. Thus, we have another 4% chance of getting lucky. But even if not, just calmly level up to level 22 following the specified route.

And if you've already scored mind control or anti-magic scrolls, or a Tome of Light or Darkness by then, it basically doesn't matter what you take — we'll kill the demon anyway. Almost any skills will look decent. If we take Luck, we get Soldier's Luck and magic resistance, all of which certainly fit into the concept of maximum damage from the first turns. Light Magic also looks good. In this case, I would definitely take Fire Protection and Bestowing Blessing to remove harmful effects. Also fits well enough. Chaos Magic may also have a place, but only if we are against Jaleb, who put everything into the ballista. Firstly, Chaos magic will help destroy the ballista faster. Secondly, Fireball reduces defense, which fits the maximum damage concept. Thirdly, summoned creatures will take 2 times more damage from Chaos magic if we take Exorcism in Summoning. Fourthly, you can get another 10% damage, as well as +2 to Knowledge, which is useful given the enemy imps draining mana. In short, Chaos Magic is also an option, but only if you know that Jaleb spent skill points in the attack branch specifically on the ballista. Then Chaos magic will work well. The main thing will be to take down the ballista, and we'll deal with the rest. Defense would look normal if you know the demon didn't build up the ballista, but the troops. Then it's a very necessary thing. At the same time, it will protect our troops from our own in case of mind control.

What I wouldn't take for Dirael against a demon is Education, because this skill is good for a prolonged battle, and we need to cut the enemy down quickly. I wouldn't take Sorcery either, as we won't manage Counterspell anyway because of the imps, and we won't be casting much since it's better to shoot with an Enchanted Arrow.

That's all for skills. I can only conclude that Dirael successfully fights demons with her Summoning magic. Most importantly — she starts much faster than Wing, which provides good opportunities to stock up on anti-magic and mind control scrolls or Tomes of Light and Darkness. Wing can only reproduce Arrow Rain with pushback if he goes into Chaos magic instead of Light. But then something would need to be done about Dark Magic.

Conclusion: Dirael is cooler than Wing even against demons! Why do people always discuss how to build that nerd?! He's frail. :)

Added after 32 minutes
Now a bit about Dirael's army:
1. If the troops aren't very large, it's better to take fairies that cast swarm. They will help kill demonesses. This is more important than slightly increasing their damage. Moreover, if they step out immediately, demons will quickly tear them apart with their attack. Another plus from them: while they are casting, they don't need to move and can cover the shooters. There won't be any Tactics. So they might extend the shooters' lives a bit along with the ents. Although against demons, shooters are just fodder; they won't survive anyway. It won't be possible to leave them good cover because troops must be sent forward. But if fairies remain, it might be possible to cover the shooters along with the ents. Or maybe they need to be thrown forward. Depending on the situation. In any case, some flexibility in tactics is preserved and the ability to protect shooters from someone being summoned right under their noses. I think this is more important than a single point of damage.
2. Dancers. We take those that hit in an area. There's nothing to discuss here — maximum damage concept.
3. Bowmasters if there is Soldier's Luck, and straight-shooters if not. In the first case, we can delay the demonesses' turn. This is important because then we can further delay their turn with the hero. And meanwhile, demonesses will be hit by fairies with their swarm, druids with their lightning, and even dragons if they can reach them. So bowmasters with Soldier's Luck rule. But if it's not there, it's better to shoot as hard as possible just to destroy the stack faster.
4. Druids with horns. I don't need Mana Channeling. It's good for a prolonged battle when the hero casts a lot and a lot of mana has to be spent. But here, firstly, everyone needs to be wiped out quickly, and secondly, the enemy has imps; they will take some mana anyway, which somewhat devalues mana channeling. And that's not all. The ballista also needs to be killed, and druids happen to have a mass spell from Earth magic. It will be very good to both take down the ballista and damage the demons. Therefore, there is only one option here — horned ones.
5. Unicorn with Resistance Aura. Well, that's clear: fewer fireballs from the ballista, fewer mind controls, and we blind them too.
6. We take more attacking trees. The main thing is a fast battle.
7. IDK. I don't know here. I haven't used the new dragon grade much. Maybe it's better, I don't know. But I prefer the old one, as it can be well-directed at demonesses if they aren't covered too tightly. The second one has some kind of uncontrollable fire.