I did it! I beat the Necromancer, first time too.
I can't believe I won - the fight was nuts, too much stuff going on for me to be in control of. I just had the idea that if I kept grinding them down with Evil Shoal and Underground Spikes, as well as Phantom images, then I might have a chance. Even so, it looked like I was a goner when I was down to 100 Black Knights, with the obelisk still with 17,000 health.
But I had whittled down most of the Necromancer's forces, and he was out of mana for casting. So I hasted my Black Knights and charged. Can you imagine my surprise when I saw that the Knights (now down to 60 or so) were doing 3000 damage to the obelisk per turn?
I won with 44 Black Knights to spare. I love this game. It's starting to look like I will be able to win on Hard in the next few days