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in the second mission, there's a cool trick)
There's a garrison between the Necropolis and the Stronghold that's separated, and it gets filled with a large army when it's first approached. If you put 7 level 1 Imps in it, the garrison will be left with those 7 Imps instead of thousands of Necrophages when it's first approached, and then you can easily defeat it, thus securing your path to the Necropolis Castle.
The final battle wasn't particularly difficult (probably because I played it after a single-player map). By chance, the Witch's Hut contained Light Magic. After some thought, I equipped my hero with the following:
1) Ring -2 Spirits
2) Ring of Sinners
3) Dwarven Hammer
4) Dwarven Helmet
5) Sar Iss Armor (Dragon Armor would have been more useful, but it's no use crying over spilled milk)
6) Dwarven Boots
7) Amulet of Skill (wasn't useful)
8) Ice Shield
9) Necromancer's Cloak
10) Clover
There aren't many buffs on the map (a flag, sacrificing 1 Imp for 2 luck, and an idol (in the Necropolis)). Since the Overlord has huge resistance and a thousand and a half Imps, there was no point in casting spells, so I dumped Black Magic, taking Machines, Attack, Defense, Bonus Damage (in vain), and Magic Resistance, and Luck with Resistance and Wide Gates, and Light Magic with Fire Protection.
As a result, the enhanced Armageddon, which used to deal 5k damage to each enemy in single-player, now had (50% Fire Resistance, 50% Ice Shield, 25% Dwarven Hammer, 2 gnomish items (40%), 15% Magic Reflection, and 15% Magic Resistance), and wasn't so scary, and in the battle, the Vampire Princes were the ones who took the most damage, but they also retreated.
For this battle against Black Magic, a Torturing Strike might be useful, but it's not worth investing in that branch just for it.
I didn't build up the army for long; the army size was 79-112-267-595-1265-1934-2420.
Calling for reinforcements allowed me to save part of the army from the upper cities, which were being attacked by the Knights. The lower right castle was the first to fall; I don't know how anyone manages to keep it. My hero (who was always ahead of Sigrif in level) couldn't withstand the first Knight in direct combat without serious losses, so he ran around with 7 Imps and mowed down his army with a ballista, Ice Bolt, Devil's Strike, and Subjugation. But at the same time, I managed to gather a spell from 3 castles.
Reading the reviews, I also reloaded at the very beginning to find the artifact for spellcasting behind the bears; 10 attempts gave me the Phoenix Cloak, which I agreed to take :)
And the Mentor is a pain; he's invulnerable. The first time, there were 300+ Lava Dragons, which couldn't be pitted against each other, and it takes a long time to defeat them with ice and a ballista, and there's no time for that because you need to either bite or defeat the Knights. By the time I finished, there were 2282 of them, so I couldn't use the Mentor, but that's okay. In single-player, I encountered Crystal Dragons, which I could have defeated, but I didn't. In the campaign, I just wasn't lucky with the guards (the price for the Phoenix Cloak). Because of the lack of access to the Mentor, all the other heroes, except for my hero and Sigrif, were useless, even though they could afford to cast a single Devil's Strike.
In the battles against Faidaen and Duncan, I had the following equipment (Dragon Ring, Ring -2 Spirits, Mage Staff, 4 gnomish items, +1 damage, Dragon Cloak, and Horseshoe; it's perfect for defeating them in the castle, but it's scary in direct combat).
Added after 15 hours 44 minutes
in the third mission, there's a trick that the main hero can't surrender in battle, otherwise it's a defeat.
in any case, when the portal to the Orange lands was guarded by three hundred Thunderbirds, I calmly reduced their numbers and retreated.
When I surrendered in a battle against the Crypt, it counted as a defeat; when I fled, it seemed to be okay.
Too bad that you have to buy Machines again after raiding him with the main hero