Heart of the Night

After starting to develop Greta again, I managed to reach level 16 before the first attack:
Nymphs 140
Dancers 109
Archers 114
Druids 55
Unicorns 23
Ents 11
Dragons 6
N 6
Z 13
K 3
Z 12
Masterful Avenger: Enchanted Arrow
Basic Education: -
Masterful Chaos: Lord of Cold, Freezing Cold
Masterful Machines: -
Advanced Summoning: Lord of Spells, Exorcism
Masterful Light: -
The Ring of the Dragon's Eye doesn't save in this battle, as the enemy's ballista eliminates all archers except 6.

I just tested it again on the first Necromancer hero - level 18, education with the bald guy, light - Righteous Fury, attack - BB and Forest Fury, leadership - Diplomacy (didn't get Empathy, it would have been even easier). After a few tries, I won with almost no losses, and didn't cause much damage to the city. The replay is attached. If I had gotten Storm Wind, it would have been even easier. On other difficulties, it's much easier - there are fewer troops and machines will move less often.