Keep building ashis and don't garrison anything that is not really crucial for happiness or defense. Keep a big stack and swing it where it is needed.

Also, use your agents. A monk for morale buff/debuff may do wonders in a fight where it is basically a face off between 2 stacks of ashigarus.

Diplomacy is something I suspect many people are not monitoring turn-to-turn in normal mode, but you really have to keep your eyes on it in hard+. Also, be aware who is allied with the clans you plan to attack, since a nasty surprise might happen when you assault one clan and their allies in your opposite flank decide it is time for war. In the beginning you can basically afford one decent stack, which is not enough to secure two borders.