I've finally found a formation which can be used for an easy win. It consits of: 1 Trebuchet, 4 Monk-Rifles, 6 Loanswoard (Veteran), 2 Yari-Infantry and 2 Katana Infantry (or Katana-Hero if ready). Its really easy:

If the opponent is going for one of the buildings my trebuchet opens fire on his units till the building is captured, than I destroy the bulding and the bonus is lost. After that I go for a building myself (while under artillery cover) or I get in formation for an attack (if he is trying to get me). First line are my rilfe-monks (very high morale), they are covered by katana infantry. The sides are covered by 2 yari infantry in speerwall formation. After about 3-4 volley of my monks I send my loanswoards and my katana infantry into the main line of his army while my trebuchet continues with open fire, lowering the moral of his units.

My general is good positioned to make sure that every unit gets his morale bonus, making the ashigaru nearly unrootable. With this tactic I've won my last 3 battles in multiplayer while losing nearly no unit (~200 loses).

I'm thinking about changing the katana-infantry with 2 ninja-units on hiding mode next to my monks, they throw devasting grenades and would stop every ambush towards my riflemen.