Necropolis and Demon.

Here's another fact: Necromancers usually don't have a lot of mana; it's rare for them to have more than 70-100. Considering the large number of Imps (if you keep them), there will be even less mana left, so the Necromancer won't be able to cast many spells. Although a Demon will also struggle with magic, and as Drake said, there's no point in developing Darkness in this scenario; it's better to focus on Light or Defense.