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. Darkness magic.
3. Demonic Gates.
4. Imps!!! This point is often underestimated. Meanwhile, they suck 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 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 controls, 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" targets, I proceed from 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 upgrading Summoning magic beyond "Advanced" has little significance for the battle with the enemy hero. 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 upgrade Summoning magic beyond Advanced and Fire Warriors.

2. Logistics and Keen Mind. It is important that the demonesses and nightmares move as late as possible. Thanks to Keen Mind, the battle will start with us either casting a general Haste and gaining the ability 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 upgrade 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 due to the aura. All that's left is to find boots and a fast battle start 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 will also take it 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 upgrade 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 those melee units that aren't the slowest.

So, all the above-mentioned skills can be acquired 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 fairly high drop probability, so obtaining such a hero is quite feasible and should not cause particular problems. Further upgrading 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, scrolls of mind control and a Tome of Darkness.

5. One skill slot remains. If we are lucky, either Darkness 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 pleasant because with an Enchanted Arrow we can cast the swarm another two times per turn, and in combination with a rain of charged arrows — 5 times! In this case, it's better to shoot at imps and extinguish them quickly so they don't suck mana. They likely won't be able to move 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 to 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 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 facing 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 sucking mana. In short, Chaos magic is also a thing, 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 out the ballista, and we'll deal with the rest. Defense would look normal if you know the demon didn't upgrade 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 would NOT take for Dirael against a demon is Learning, because this skill is good for a prolonged battle, and we need to cut down the enemy 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 Darkness 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 push out immediately, demons will quickly tear them apart with their attack. Another plus: 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 life of the shooters 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 together with the ents. Or maybe they'll 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 something being summoned right under their noses. I think this is more important than a single point of damage.
2. Dancers. We take those that deal area damage. 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. Meanwhile, fairies will hit demonesses with their swarm, druids with their lightning, and even dragons if they reach them. So bowmasters with Soldier's Luck rule. And if not, it's better to shoot as hard as possible just to destroy the stack faster.
4. Druids with horns. Channeling is not needed. It's good for a prolonged battle when the hero casts a lot and has to spend much mana. 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 destroy 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 offensive 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.