I found you need to divide your starting non "horde" troops up to make sure they are not in the same stack. With the first city you stay in half(?) of the special horde units go back to being peasants, then half of whats left on the next city,and then the rest on the last.

Under my campaign I had my Gen and a couple of peasant to take the last city , against ERE who didn't have a large garrison but it was of decent quality. I had to delay taking the city until I could generate some decent support.