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Basically, it's not as difficult as it seems. By the end of the first month, I placed Unicorns (Griffins weren't an option), then I stayed near the school, saving up for each hero's upkeep. They run around and clear things out. They even cleared out slow Hydras with a few Ifrits in the underground, but 5 Phoenixes near the city's mine took them down one by one, and I had to load twice for the last one to prevent a hero from being defeated.
By the end of the 3rd month, I had 5 heroes and 100 Elves. I was just chilling. One hero had Order 3, the other had Nature 3, and the 100 Elves get buffs and take down many enemies.
Some Necropolis heroes are taken down with Holy Words. 30 Unicorns are waiting, but they aren't needed yet. Only 2 neutral dwellings have been taken, but they will help take more.
Added 16 hours 59 minutes ago
So, starting from the 3rd month, when 120 Elves had accumulated and were recruited, the clearing process became easier - apply buffs, and they start to perform. I captured all the Elven dwellings, a total of 10, resulting in 6 full growth rates for the Castle (42 Elves per week).
I needed to reach the experience limit. There's enough experience on the map, but to avoid running around collecting small amounts and engaging in long battles, I found 4 levels around level 16 for the Barbarian and 13 for the others (the Barbarian fought Elves alone when there was water, or even completely alone when there were Devils). Then I noticed another one. The Barbarian quickly reached the limit, then the Tactician reached the limit, and I leveled up to 15, transferring artifacts and nectars from one hero to another, and completed the Coliseums. The Necromancer was the most difficult; I used 6 nectars on Black Dragons, and even then, the Armor of Order and Mirror of Illusion allowed me to reach those levels.
Then I completed the neutral altars and artifacts, resulting in:
Barbarian - full combat, life 3-2-2-3, Order 1, Death 1, Chaos 1-1-0-1 (unfortunately, no altar for level 2), artifacts: 2 HP rings, Amulet of Failure, Halberd, and Armor of Order.
Lysander - Tactics 5-4-4-3, Combat 5-1-0-0, Life 1-1-0-2, Order 2-1-1-0, Chaos 1, with artifacts providing +50 attack and defense, an invisible Amulet of Life, a Helm of Leadership, and a bit of morale, in battle, all Elves have -5, not -11.
Necromancer - Combat with 2 resistance, Life 1-1-0-1, Order 3-3-1, Death 4-3-3-4, Nature 2-2-0-0. And without a spellbook, he knows Vampirism, but still, he has a spellbook and Armor of Chaos, the rest is minor.
Elf with Combat 2-0-2-0, Life 3-3-2-1, Order and Chaos 1 each, Nature 5-5-5-0, no level 5 summoning, but the spellbook provides it.
Mage with Combat 2-3-2-0, Life 2-1-0-1, Order 5-5-5-0, Death 1-1-0-0, and Chaos.
And the Rogue with Combat 5-5-2-0, Scouting 5-4-0-3, Life 1-1-0-1, Order 2-2-0-0, and the first Chaos.
In the army, there are 682 Elves (lost 11 during the game, 4 of which were resurrected) and 252 Ghosts trailing behind.
I can attack the garrison (8th month and 3rd week); the Necromancer and Mage know the Portal spell, so they can join the Elves with buffs. Everyone is on nectars, 3 with 20 defense, the rest with 10. 2 potions of anti-fire ran out at the Black Dragon Colosseum; the Barbarian won't have anything to defend with. The nearest Temple of Nature and Fountain of Might are far away. They will be taken on the second attempt.
The first 2 garrisons were taken without problems; in the third, there are:
3 heroes
36 Phoenixes
216 Ifrits
369 Ice Demons
468 Imps
Added 16 minutes ago
Hmm. I defeated it on the first try, costing approximately 3 nectars. The Necromancer used Weakness, Curse, the Elf used Magic Bolt with -hero per turn, then Dragon Strength and Manticores separately, the Mage used general Slow with Blind, the Tactician used single-target Killers, and the Rogue used Enthusiasm from a scroll. The Barbarian just hit with a Stunning Mace. Approximately 3 nectars, -1 potion for damage and HP, and a potion of cold that didn't work. A second attempt wasn't needed. 138 Unicorns were not needed.
Added 7 hours 7 minutes ago
Phew, I defeated the Reborn Bru in the final mission.
He's just a monster. I took the castle (he's standing nearby, clueless), locked myself in, and he attacks: 9 Black Dragons, 6 Phoenixes, 50 Ifrits, 6 Beholders, 35 levels, insane stats.
I can't afford to bribe him, I can't use the Portal, all except the Verjak are under Armageddon after the general cancellation.
So, finally. Only his weak resistance could be broken. I managed to cast Blind on the hero. While he was sleeping, the surviving Necromancers cast Vampirism, Blessing, Aura of Fear, even summoned Manticores and gave them Dragon Strength and Mirror of Pain. The Barbarian and Manticores defeated all his troops, but I couldn't re-cast Blind or Swarm of Wasps.
He woke up, cast Armageddon, and all except the Verjak fell. He had removed the nectars beforehand.
Then, a magical duel - he casts Dragon Strength on himself, the Verjak removes it. And so on until the mana runs out.
Then, the Verjak can't defeat him. He regenerates everything. The Verjak's Fireball won't work, it deals little damage.
But finally, the potions helped. Standing on the tower, having Blessing, Fire Aura, Bloodlust, Ice Blade from a potion, Spiritual Armor, Poison Attack from a potion, Retribution, Accuracy, Amulet of Death, Killer, his own regeneration (Mage of Life, what else?), Enthusiasm from a potion, he was able to defeat him. Without Fire Aura and Poison, it wasn't very effective - a close fight, and until the protective Blessings wore off, he still had to heal. But here, he finally defeated him. Hooray. The Mage will have to complete the mission for a long time.
Added 9 minutes ago
And actually, I messed up a bit.
After completing the quest, I teleported the Necromancer to Kryl's city, but I could have waited and jumped to the correct one; in fact, he's trapped there (although he'll find a way out).
It would have been easier with the Necromancer than with some with -11 Spirit.
After the victory, it said that the orange one was defeated (on his turn, during the siege), but he continued to move. He will stand for the next turn; I'll finish him off with the 2 remaining castles. The Barbarian did it all.
Added 41 minutes ago
And here's a strange thing: it turns out there are quest gates; I didn't see them. They require all 6 heroes and no higher than level 20. I leveled up the Verjak to 2 in the Coliseums; is that it now?