Has anyone played through Len’s mission in Heroes of Might and Magic II?

I also bought them in the tavern. After collecting all the fairies from all forty heroes, and also from the castle, I only had 40 fairies.
The first three Necromancers only have the speed magic spell. But the second wave of Necromancers always comes with the Cold Ray spell.
It seems that this mission doesn't have a universal walkthrough, and for the most part, everything depends on luck.

Luck has nothing to do with it. Every time you start the game from the beginning, the number of fairies in the heroes, the magic, and everything else is generated randomly. You just need to find the optimal option for yourself by restarting the game several times, so that everything suits you.
However, there is no single algorithm that works for any starting conditions, and that is true.