I would probably choose Skeletons, Vampires, and Liches. There are many Skeletons, and with Battle Battlements + a necklace, they hit VERY hard even now. Vampires attack without retaliation and drain a lot of health (don't think of anything else :)), they are a very annoying opponent. Liches are good archers and casters; it's best to kill them early. You can't deal critical damage to Necromancers with just archers and druids (and Chaos magic, of course). If Berserk or Suggestion is cast (with upgraded Sorcery, the Necromancer will have time to cast it once or twice), it will be a disaster. With the Light faction, as well as with the "Light Magic" perk itself, you might not be lucky (the chance is only 4% due to the incompetent developers).
It's better to put something on the arrow that hits only one target but reduces armor, slows down, etc. In this regard, Dark magic and Rain (even against Necromancers) will be no worse than Chaos, and you won't have to worry about hitting your own troops in the front line.