It took me a lot of effort to defeat a Necromancer hero in a duel! Probably on the fourth try. After completely revising the tactics and respecifying the hero. I took the second type of army, without any buffs. The hero is a mage, path: Tears. Skills:
Primal Magic (max) -> +30 mana, general speed, time stop, explosion.
Magic of Light (max) -> general healing, summon fireflies.
Tactics 2, skill for +5 initiative, archery 2, and some minor things, like increased defense, etc.
Tactics: on the first turn, use Tsunami and attack with all troops. Due to the enemy's counterattack of 3, it's not easy for them to attack; they have to focus, but it's not very effective. Kenshi destroy the Lamassu (although not immediately, but gradually); the enemy destroys them. I cast Time Stop on the vampires in advance so they wouldn't move. The attacks were not very effective; his Sphinxes died, I blocked the archers and almost killed the liches with the spiderlings, but it was more difficult with the rest. But I had archers, and they were powerful, so I defeated them with archers. In the end, I had 5 Medusas and all the Snow Maidens left.