Late-game Town Enhancement.

For several years now, I've been trying to figure out how to defeat an Orc army using Dark Elves. The problem is that Summoning and Chaos spells become significantly weaker in the late game, and the power of magic doesn't increase as much as the strength of the troops. So, the question is, how do you defeat an Orc if the game is very drawn out, and you don't have access to powerful Dark Elf spells other than Berserk (which you can borrow from the Necromancer faction)?