Destruction magic, although the coolest for the Elven Alliance, becomes less decisive with large armies. If it reaches that point, you can choose a hero with army enhancement, like Iranna or Vaishan. Although dark magic isn't very effective against necromancers unless you have Sandro's cloak. You need to figure this out on your own.

Summoning magic is a good option against necromancers. Not long ago, I played against demons as the Elven Alliance in the late game. We reached very high levels, and my hero had a magic power of around 45-50. The opponent had cursed ground, making it useless to summon phantoms. I summoned a phoenix, and it destroyed all of the opponent's archdevils (about 76 of them) in its first turn. It was just luck, though. It would have taken Erin with her witches many turns to achieve the same result...