I have done single player with only archers and axethrowers. It is actually harder to do the single player with only footmen and grunts, because footmen can't attack ships or dragons. I have also done the single player campaign with only peons and peasants. If you want a real challenge, try orc level 9 with only peons. You can also use units that can't fight, like oil tankers, zepplins and transports.