Kill the Daimyo.
And you need to have at least 2ooo soldiers in every province that meets the hords provinces.
And like he said before me up there.....charge them off the map
I just finished the expert campaigne with only Ashiguras units (ashis, musketts, arq´s) Yari Samuraij´s and Archers......and it was a hard thing.
Best of luck to you.

------------------
May the honour be with you all....
-LinkSan Daimyo of The Chain Clan