Your mention about the druid problem actually suggested a solution to me: the swarm. There's only been a few of these so far, to my knowledge, but I can't think of a good reason why you couldn't have some sort of 'zombie swarm' or 'skeletal swarm' that moves and acts like a single entity, and increases in stats & size as the necromancer gets more powerful. Outside of combat you could declare that they're operating separately, and you can get in some handwaving by saying it's too difficult to direct them individually against separate targets, rather than sending them all in after a single foe at a time.