How to defeat Deleb on "Old Friends"?

Regarding mana consumption. How many of these demons will there be, and will they steal all the mana?

The question is different. How much mana is needed so that the demons don't steal it all?

I've always considered Chaos to be an idiotic choice for Necropolis. The only exception is Ravenka, as it's built-in for her. I don't think it's worth sacrificing so many levels for just one ballista.