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I don't have time to read all the pages, so I'll describe how I did it:
(1)
On "Hero" difficulty, take demons as a reward, hire a second demon hero, hire imps, and with the second hero, capture the imp lair on the right, and transfer the army to Agrael. With Agrael, attack the elf, proceed without stopping to the castle, and instead of capturing it, turn north, go through the mages near the crystal mine to the earth elementals.
It's better to use the Curse spell against them; it's good if you have Devil's Strike; summon all stacks through portals; you can split the army for more stacks, cast the spell, and throw meat at it. If you took a Phantom as a reward in the previous campaign, it will be useful here; with luck, you can defeat all the elemental stacks with 3 stacks of phantoms.
Next, go to the northwest castle; you should encounter the local hero; he doesn't have many troops; with skillful play, you can defeat him. Result: the northwest castle is captured on day 6.
(2)
In parallel, with the second hero, you can gather demons at the bottom, collect money, build demons in the castle, and hire them, go to the castle to capture it. I advise you not to give simple demons to Agrael; an extra 5-9 won't hurt. Visit the experience stone; with a high probability, you will find the Curse spell, which will help you kill the garrison; but if you encounter a hero with a ballista, there will be no problems.
It's better to take the castle on the 1st day of the second week; then the result is: you have 3 castles (1 demon; 2 elves) against 1 castle (elf) of Gilraen; and the druids won't bother you for another 7 days.
(3a)
If you failed to save the army, it's better to use the week in reserve to eliminate the minor heroes (collectors - a distinctive feature is less than 10 units in the army), while hiring an elf hero in the upper castle and an army from there, give the army to Agrael, and send the elf to capture the mines. With Agrael, go back, killing heroes, and capture the gold mine near the lower castle (you can also take the 2nd one); the third mine is near the upper castle; rebuild the upper castle; in the next week, take the army and either attack the druids or try your luck with Gilraen. The rest is a matter of technique. Result: while you are running around, in two weeks, Gilraen will have a normal army, so the game will be a little longer, about 4-5 weeks; and if, God forbid, he goes around to the right and captures the lower one, it will take up to 6-7 weeks. It's better to act like this: on the 3rd or 4th week, go from the top to the observation tower to see when Gilraen will come out. No matter how many times I've tried, the computer starts developing the northwest castle first, so Gilraen sits in the castle for a couple of weeks until he has an army. Return to the northwest castle, wait, and Gilraen will move on the 3rd week after capturing the northwest castle (sometimes on the 4th) down and to the left to the northwest castle (you also need to scout everything in advance at the bottom). You need to immediately meet him and defeat him; otherwise, he will gather 500-1000 dryads and 200-400 blade dancers. Then, replenish the army and capture the 3rd castle.
(3b)
If you have saved at least some army (mainly in cases where Agrael is leveled up through the ballista), then immediately go to capture the third castle; Gilraen should not have a huge army yet, at most, something like 30 archers, 20 druids, 1 green dragon, etc.
And if you're doubly lucky, half the army will be in the castle, and Gilraen will be leveling up at the bottom right. Capture the castle and wait for the elf to come, saving you time.
Result: 3 elven castles are captured, the hero is killed, victory in 12-16 days.
(proverb)
I implemented option (3a), leveling up Agrael through Urgash's gate, starting the map at level 16, and finishing at level 19, completing it in 46 days.
I implemented option (3b), leveling up Agrael through the ballista; as a result, I didn't even hire elves and managed with the initial army in 16 days.