Can Necromancers even be defeated?

It's impossible to say that any race has a decisive advantage. All races are different, yet roughly equal, which is why the third installment of Heroes of Might and Magic remains so popular. I maintain that the Tower faction can effectively counter the Necropolis. Titans are stronger than Dragons, Core Nagas are stronger than Evil Knights, and it's difficult to drain the life from Gargoyles and Golems. The Tower has its own strengths; you just need to know how to use them.