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

Luck has nothing to do with it. Every time you start a new game, the number of fairies in Heroes of Might and Magic and other elements are randomly generated. You just need to restart the game a few times to find the best option for yourself, so that everything suits you.
However, there isn't a single algorithm that works for all starting conditions, and that's true.