Late-game Town Enhancement.

For several years now, I've been trying to figure out how to defeat an army of Dark Elves. The problem is that summoning and chaos magic become significantly weaker in the late game, and the power of magic doesn't increase as much as the troops' strength. So, the question is, how do you defeat an Orc when the game is very drawn out, and you don't have any high-level Dark magic spells other than Berserk (which you can borrow from the Heroes)?