Been a while since I did that campaign but I will tell you from what I can remember. I think you have to inflict so many casualties before they leave the map. Is this correct? Well anyways the whole idea is to entice there army to stop and force them to fight. I remember getting it on the first try and what I did was force my cavalry to be vulnerable to get them to turn around. I had 2 units of cavarly threatining there rear while the rest of the army was marching there. 1 unit of cavalry got tied up and had spears approaching on the flanks and the other unit was able to escape. I routed the tied up cavarly before the flanking spears could inflict heavy damage. After that it was basically straight up tactics as they were forced to fight.