Haven tactics.

If we consider the scenario of necromancers versus the forces of Heaven on the lands of the outcasts, then judging by your comments, you seem to underestimate the necromancer.

Well, and you seem to overestimate him a bit)))

Secondly, a stack of 1000 zombies under Orson can easily withstand a charge from 300 Axel's knights of Isabel.

Those are some scary numbers. I've never had so many creatures.
And in general, who would attack zombies with paladins?
I would first get rid of the necromancer's strongest stacks - vampires or liches. Maybe even messengers. And if anyone survives, there will be very few of them left. Axel's paladins are a force!

Thirdly, while you're busy coordinating the paladins, the necromancer will be subduing and berserking everyone else, and the paladins will most likely be wiped out after their first attack.

Depending on the situation. Firstly, the paladins won't be wiped out easily. Half of the undead will die from counterattacks. Secondly, mass dispel magic is always available and works well (usually). Plus, I always take artifacts that increase magic resistance. So you're overestimating dark spells.
After the necromancer casts Berserk (the AI always loves to do this). On my hero's next turn, I always remove it with dispel magic, and the Berserk effect doesn't work.
Thirdly, even considering the spiritual connection, the necromancer has very little mana and can't cast many spells.
Fourthly, Heaven is strong not only with paladins. Archangels can also severely damage one of the enemy stacks with their turn, archers without penalties will destroy skeletons, and griffons will swoop down from the sky on someone.