Oh man... this topic reminds me of the funniest game of Mage Wars I've ever played. A friend and I had agreed to try out building custom books for the first time. I chose the Forcemaster as my mage (having never played it before), mostly because of the cantrip weapon. I decided to build a solo book by compiling every powerful buffing enchantment and piece of equipment, as well as some typical mind control stuff (I had absolutely no creatures or conjurations). When my opponent announced he was playing the Necromancer, I honestly figured I was screwed since all of his creatures possessed psychic immunity.
And then we began the game... The necromancer stayed on his side of the field and started amassing a horde of zombies. I decided not to press him, as I wanted to see how much punishment my mage could take. Instead, I simply geared up. After ~8 turns, I finally moved to the center of the board, and he attacked with all 10 of his creatures. Hilariously enough... they couldn't actually touch me. At that point, I had 7 defenses, 5 armor, and a whole lot of other defensive perks.
At this point, I could have simply ended the game, but I was just having too much fun watching my opponent's mounting frustration (he had apparently forgotten to add any dispels into his book, so his only counter was to play more and more zombies). Instead of going for the necromancer himself, I simply cut down his minions as fast as he could summon them (he had to run out eventually...). I even let him build up a max-strength Shaggoth-Zora (it took me all of three turns to cut it to pieces, and it only dealt 4 damage to me).
After 2 and a half hours of this, my opponent finally quit (with an expletive-filled rant that I won't repeat here). Sadly, he has refused to play me ever since.
So yeah.... turns out one of the easiest ways of beating a zombie necromancer is to just buff yourself up as much as possible, pray your opponent doesn't have a purge or destroy magic, and just wait patiently until he rage-quits.