I have never played necromancers, but I have faced many low/mid ranks with hero armies who just wiped the flour with me. I think (as most of you have said) a necromancer should be able to keep an army of reasonable strength, but there should be some ways to control the army power with the help of elders, so their younger enemies can find a chance facing them.

I was thinking about a few solutions. One could find a mixture of them as the best answer too:

1. A level X char has a level range (Let's say X-5) access to summoning under control zombies.
2. A necromancer can have an anchor like spell (Maybe not even an spell, just a passive factor used in the background) to protect summoning his/her zombies. The lower level and fewer zombies you have, the higher chance of keeping them. Might even make a necromancer being able to have one same level zombie impossible to be summoned.
3. Others out of PK range, can summon the zombies if in same area as the necromancer. So, force the elders to walk side by side of their lower level friend to be of any help.
4. Make summoning out of range zombies a real pain. Huge amount of mana, zombies who fart on summon and make the summoner ill? (I don't know what is the elixir you all talk about, but it seems the solution I am talking about, if it does give reasonable downside)