First, as BoardGuest8088888 said, you should never be in that situation in the first place. The strength of Necropolis, as any one knows, is numbers, massive, massive numbers. Between the growth rate, creeping, and with some luck, creatures structures that can be used with your undead transformer, your army should have the advantage of superior numbers. Second, if you built your hero correctly, you should be able to make use of all the necro's casting strengths, such as the mass curses, puppet master, frenzy, raise dead, as well as the summoning spells, especially conjure Phoenix, and in addition, have his attack and defense buffed (there is some debate about the usefulness of defense with a necro army), and if you were patient enough to acquire luck, you should emerge victorious. This isn't to say you will never lose. Any town can be beat by virtually any town on any given day, but the odds should be in your favor.
![smile_teeth :D](/forums/images/smilies/smile_teeth.gif)