Just thought I'd add my strategy to the list here. I did it with sprites, dryads, demonesses, horsemen and elven archers. My wife was the demon one, so and I placed that sword which gives 30% extra attack against undeads in one of her sword slots. I also used the ring which made the female fighters better.
Basically, on my first attempt, it was the necromancers and all the resurrection of enemy troops that killed me, so this time I decided to kill the necromancers first. The demonesses have a skill that allows them to swap the position of any two units on the arena, which I used to swap the necromancers with some bone dragons which had flown down to my area of the screen.
This ensured that the necromancers were kept busy, and was unable to use any ranged magic. They also died fairly quickly, and like others here, I necro-called them immediately to prevent them from being resurrected by the enemy. Same thing with the black knights - the first time I played they were resurrected to what seemed to be full strength three times, this time I only had to kill them once.
During the fight, I mostly concentrated on destroying the troops. I used the dryads to create plants, and used the reflection spell to get an extra set of sprites (I had over 2000 of them, so the summoned group was quite large).
I also found the ice orb extremely useful. It basically bounced back and forth between the obelisk and enemy units down in my area, dealing extreme amounts of damage (as the rolling-distance was so long). It took out more than half of the obelisk's health that way, and certainly helped thinning out the enemy ranks as well.
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