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The most difficult missions (in my opinion, of course):
People: all are very easy, only the 5th mission is at a more or less decent level.
Inferno: the 3rd mission is not easy, the 4th is quite difficult, and the 5th is also difficult (you won't have time to quickly reach Tieru - dragons will multiply, and nothing will save you).
Although the 3rd mission is not as difficult as some say. Here's how I completed it (as a Warrior):
I quickly captured the Elven town, by the end of the first week, hired an Elven hero, started exterminating Elven mobs to add them to my allies, and began quickly building dens for succubi, archers, and druids. Then, around the 2nd month, the entire Inferno Castle was ready (I'm talking about the dwellings), and the Elven one was up to Ents. I led Agrael along the road to the northwest (I don't remember who defeated the Phoenixes on the road), where I met Gilraen. I fled to Inferno, and he went to attack my Elven Castle. I had: an Elven hero with Skilled Avenger, Enchanted Arrow, Arrow Rain, Plague, Ice Bolt, 30-50 Master Archers, 20 Grand Druids, 20 Succubi, a few newly redeemed Ents, a group of War Unicorns, and Imps (I don't remember how many). Gilraen had: 50 Master Archers, 20 Grand Druids, 100+ Death Dancers and Dryads, 10+ War Unicorns, and 6-10 Ancient Ents.
Somehow, with huge losses, I managed to defend the Fortress, first killing the Faeries, Archers, and Druids, and then the Death Dancers and Unicorns (thanks to Ice Bolt and Arrow Rain), and finally the Ents. Then there were more attacks, but already with dragons. I captured the Fortress in the northwest, stationed Agrael there, repelled a couple of serious attacks on the southern town (the northern town was changing hands several times in the meantime), after which the computer ran out of steam, and I easily defeated the Druids in the center, leveled up Agrael, and finished off the computer in the northeastern town.
Necropolis: all missions are not difficult, except perhaps the 3rd and 5th (the 5th is due to the battle with Godric).
Necromancer: the 2nd mission is not easy, the 3rd mission is HORRIBLE!!! Capture 7 enemy towns, kill a bunch of leveled-up heroes... And you also need to hold your two Castles, which are being attacked by hordes of heroes of all nationalities. Only Railag was leveled up, and Shadia often couldn't repel attacks from enemy heroes.
Ranger: the most difficult campaign. The 2nd and 5th missions can probably only be completed by zerg rushing: if you don't win quickly, you will die for a long time.
Mage: the 2nd, 3rd, and 5th missions, especially the 2nd (to storm the Mage Castle, I had to send an undead army from 2 Necropolises, which had accumulated over 6-8 months, to attack it) and the 5th (by the time of the battle with the Lord, almost the entire army had died, especially Zehir's, because I learned about the power of Banish when there were already 3 perks in the spell).