Summoning magic is a good option against Necropolis. Not long ago, I played against demons in a Team Edition game. We reached very high levels, and my hero's magic power was around 45-50. The opponent had cursed ground, and summoning phantoms was useless. I summoned a phoenix, and it destroyed all of the opponent's arch-demons (about 76 of them) in its first turn. It was lucky, though. It would have taken many turns for Erin with witches to achieve the same result...