Auto-translated
I think it would be helpful to move on to the structure that is already recommended on the forum.
I'm sharing my experience of completing the final battle.
Before freeing the dark witches from the dungeon, it would be a good idea to use the services of mercenaries; otherwise, you won't be able to avoid significant losses.
I searched the entire map until I discovered that the "house by the road" is the same hut where you receive the quest to find 55,000 gold. You need to return to it and pick up the archers in one and the orcs in the other.
With the witches, it's easy to storm the Dark Elf Castle.
When the flying saucer with the Martian arrives, you can take the troops that have accumulated in the captured Dark Elf Castle, add the freed witches and black dragons to them, equip the Martian with extra artifacts, including the Tome of Light Magic, and send him to storm first.
Although his level doesn't increase beyond the second, his stats are high enough to last 5-6 turns and Divine Retribution will eliminate half of the succubi. Besides, he is a necromancer!
After this assault, it will be easier for Zehir. I easily defeated Biara with his own faction's troops (with artifacts, of course), without phantoms or mind control.
It is important to have time to transfer the Martian to K'tharr within a week; otherwise, he will attack Zehir again with four titans, losing all his artifacts and army.
P.S. When I played the mission on "Veteran" difficulty, I noticed that the monster squad in front of the castle, which is given as help to Zehir, doubles in size in a month if you don't take it immediately, from 120 to 228. At the same time, Biara does not receive reinforcements. In general, Zehir needs a squad of 4 with 120 units to win, but 228 archangels can eliminate all the succubi at once.
Of the spells, Phantom and Divine Retribution are the most effective. I didn't use Mind Control because I didn't want to spoil the effect of the set of dwarven armor with the Tome of Darkness Magic: before the battle, they cast Stone Skin and Dodge (general spells).
Willy and Frida coped with the enemy just as easily as before on "Veteran" difficulty (don't forget to provide Frida with artifacts and spells in advance), but there were problems with the orcs. It is not possible to stop them; as soon as I hover the cursor over the icon, a sand timer appears, and then Kujin immediately enters the battle. Moreover, if Kujin loses, the computer counts it as a defeat, even if Gotai wins. I had to turn on auto-battle; in auto-battle, Kujin won the game, using not only Horde's Fury but also Frightful Roar. I didn't think of using the second roar myself.