It sounds like the problem lies in your strategy not your tactics. Play the game at an easier level and autoresolve the battles (I can hear the sniggering already), the idea is to see what strategy works not your tactical ability.

This is a very quick way of finishing a campaign, at hard level it can take 40 minutes to win as the Mongol Horde and about 1hr 20 to win as Hojo on medium.

Make as many alliances as possible, expand towards one end... if you are in the middle I tend to go left as there are less daimyos.

If I go right its with a large army and I pretend I'm the horde, so stay mobile and hit as many castles as possible and sell off everything. Don't seige just let one of the opposing clans do this.
Use a non-heir general who you can kiss goodbye.

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Victory first, Battle last.