It’s precisely the divine power, as mass acceleration can be taken as a perk from the attack skill (although, then you lose fire arrows and the ability to use a super ability). Still, it’s risky for a demon to take light, unless the guild has divine power and it appears early.
And against a Necropolis, you can take weakness to darkness and maybe a seal, as Necromancers don’t always have good mana, even with spiritual connection. Vampirism might also appear – this is good against Berserkers and Subjugation.
Regarding a Necromancer-summoner – I doubt it. At least, using a Phoenix against a Demon isn’t very effective; there’s a risk of encountering Cave Lords, and there might be several squads of them.