I finished this campaign on hard so here are my tips.
forgive me if i get the clans name wrong as i forgot their starting areas.
The clan you have good relationship with at the start (trade and good standing) is called the Amako. You will have to dishonor the treaty by attacking them at some point early otherwise they will attack you along with their ally.
1)Train a decent amount of medium bunes 2 or 3 with 1 or 2 bow kobayas.
2) go to war with Amako to the west that have two provinces. this will cause you to war with their ally Shoni on Kyushu.
3) Use the navy you trained and block the crossing that leads to your regions/captial (formally Amako regions). This disables enemy from crossing to the main land and attacking you from the rear from Kyushu. This enables you to war with other clans on one front leading you closer to Kyoto to concentrate on your victory campaign (I'm assuming short campaign) note: at all costs do not let this navy get defeated because once you do..a Shoni full stack army will cross into your lands and you'll have a headache.
4) train lots of trade ships and capture as trade plots as much as possible.
5) I found that Kikkawa is a reliable ally if you treat them right you can use them as a buffer zone so you can capture regions near them thus enabling you to control their expansion. I found in my campaign that they exapanded all the way to Kyoto and then later in the campaign they decline as other clans near Kyoto fight back.
6) don't even bother and waste time capturing Kyushu, fighting the Christian clans.. Capture Tosa the Chosokabe island instead.
7) if possible and if you like Christianity better then you only need 1 Euro trade ship to block the crossing on the mainland.
Mori is by far the easiest for me, 1 front to concentrate on, superior ships (bonus if you convert to Christianity and get the Euro trade ship) fairly rich surrounding regions particularly the province eastward has a gold mine and finally its not too far from Kyoto.
The ship block crossing tactic is by far the most helpful it works and makes the campaign very easy.
"Maybe someday we could become friends. Friends who ride majestic, translucent steeds, shooting flaming arrows across the bridge of Hemdale."