Light or Dark magic.

2Shogun
IMHO, what don't you like about Darkness against Necropolis? Okay, so you can't use Plague and Forgetfulness, but what about everything else? Against Necropolis, the point of leveling up Darkness, I repeat, is because of the "Darkness Weakness" skill, but there's a catch... if the Necromancer also levels up Summoning (which happens quite often), it will be a huge problem. So, if you don't want to take risks, I agree, you should level up Light. In my opinion, it's always a good choice.
P.S. I usually play as Haven, mostly with Axel... if your opponent is reasonable, he will take out the Paladins first. If he doesn't, then it won't matter much for the Behemoths...
Seraphims somehow always manage to cast twice (not in full force, but still, the damage from Behemoths is impressive)

Added after 4 minutes
I agree with Fireball
I also mentioned in another thread about the duel between Vittorio vs. Axel, where the upgraded Griffons with Stone Skin and Vampirism completely destroyed Axel's army (and with Blessing and Smite, it's an invincible squad).