As Amp says , it's a tactic that wAs used 10 years ago rush 2v1 and rout them quick , but in the original shogun war the moral hits hurt more and would sometime rout ur team mates full army even if he hadn't even lost any men.. Fun times:p
But to counter it u need to hold ur lines keep a solid formation and hopefully u got a good team who moves fast and sends units to protect ur weak spots while also tryin to flank the enemy as well! just the basics really..
As stated u really need to post a replay for more detailed help etc..