Stealth grants experience – 25% of the experience that can be gained from defeating this enemy (I won't go into details about how it works).
Antimagic also removes the effects of previously consumed potions, including those of immortality. The hero cannot be killed or weakened by magic, but he also cannot be strengthened by magic; moreover, archers or melee fighters will easily defeat him, and he will not resurrect with Blessing.
Resistance only reflects bad enemy magic.
(although I didn’t watch the battle itself)
This is exactly what I mean.
Life Magic and Death Magic grant the hero innate vampirism.
In addition to summons and illusions, a Mage of Order has hypnosis – imagine hypnotizing a large group of powerful monsters and using them to destroy the entire enemy army!
And in general, I have a guide for Heroes IV in my signature, but it was an older version. I recently updated it and posted it here: /topic/at/915689/ – many questions are answered there.