Zybr, taking your advice to heart, played two more "Broken Alliance" battles on "HARD" difficulty against 5 "STRONG" AI opponents.
The first time wasn't too difficult, and the second time was easy. When you make the right moves, the opponent doesn't stand a chance.
On the first turn, I upgraded Zenda with "Education," and on the second turn, I upgraded her with "Logistics." These two simple upgrades seemed to solve everything. In the last game, I finished in just 3 months of game time, with a total of 58 saves.
Here are the stats for Zenda II and Zenda III. I didn't follow all of your advice, as I was playing against the computer. When playing against a human, I need to consider your other suggestions.

Zenda II, a Necropolis mage. She received her second medal.
Level 29. Mana 390. Physical Strength 11, Physical Defense 17, Magical Strength 29, Magical Defense 28, Luck 13, Leadership 2.
She destroyed 2 MEGA armies. The first army belonged to Azure Stronghold, and I gained 102,217 experience points in that battle. The second mega army belonged to Green Inferno, and I gained 166,335 experience points in that battle.

The skills were acquired in approximately the following chronological order:
1. Education.
2. Logistics.
3. Agony.
4. Drain Life.
5. General Agony.
6. General Drain Life.
7. Training (as I urgently needed a second Hero).
8. Protector 1.
9. Protector 2.
10. Dark Magic 1.
11. Counter Strike 1.
12. Counter Strike 2.
13. Aggressor 1.
14. Dark Magic 2.
15. Stop Time.
16. Summon Earth Elementals.
17. Subjugation.
18. Summon Dark Elementals.
19. Terror.
20. Dark Magic 3.
21. Poison Cloud.
22. Dispel.
23. Find Path.
24. Steal
25. Reinforcements 1.
26. Reinforcements 2.
27. Luck 1.
28. Luck 2.
29. Aggressor 2.

Added 6 minutes ago
Zenda III. The 3rd medal in the same battle as number 3. There was only one General Battle, and oh, what circumstances! Again, against Azure Stronghold, and he had some kind of MEGA army (350 Furies, 330 Crushers, 70 Cyclops, and so on). The entire army was destroyed because I had the "Shackles of Fate" artifact. Zenda's losses were only 15 Pikemen. I SWEAR! Once again, I am amazed by the racial ability of Necropolis.

Zenda III. Level 23.
Skills:
1. Education.
2. Logistics.
3. Agony.
4. Drain Life.
5. General Agony.
6. General Drain Life.
7. Training.
8. Protector 1.
9. Protector 2.
10. Dark Magic 1.
11. Counter Strike 1.
12. Counter Strike 2.
13. Reinforcements 1.
14. Dark Magic 2.
15. Stop Time.
16. Summon Earth Elementals.
17. Subjugation.
18. Summon Dark Elementals.
19. Dark Magic 3.
20. Poison Cloud.
21. Reinforcements 2.
22. Find Path.
23. Steal

Artifacts: Shackles of Fate, Archmage's Bracelet, Crystal Lens, Stone Fist Clan Scrolls, Prophet's Pauldrons, and some others.