The fight isn’t terribly hard if you split your forces properly- make sure you have at least one fire Pokemon head near either corner banner, and you should be able to set up good defenses to topple Motonari’s forces when they come after you and the banners. If you attacked with only one Fire Pokemon, it becomes considerably harder- pick a banner to hunker down at until Motonari has only the two Pokemon on the other banners left, then sweep through as fast as you can and take them out with your Fire-type as well.
Motoharu likes to descend on the leftmost banner, backing up one of the two Sewaddle-using Warriors, while Motonari’s Snivy runs after the far-right banner. Snivy’s the less offensively powerful of the two thanks to Motoharu’s use of Mighty Blow and Melee, so send more of your force his way if you’ve got an uneven spread- and make sure you take out one of the two Pokemon there quickly. Pansage is more dangerous than Sewaddle, but without Sewaddle nipping at your flank, Melee won’t get triggered and you don’t risk losing healing items to Bug Bite’s side effect, so your better choice depends on how you prepared for the fight.
Oichi works well if you park her Jigglypuff on the central banner- all of your opponents use melee attacks anyway, so she never has to leave it, and the healing will keep her Jigglypuff up as long as you like, ready to unleash her Warrior power to patch up your entire force if need be, regardless where they all stand. Your ability to boost your Eevee’s range drastically for one turn means that you should probably set it up on the far-right banner, especially if you can pair it with a fire-type, who may need Eevee to weaken the Snivy so it can be brought down quickly.
Once the battle concludes, you should see all of Motonari’s forces marked as recruitable if you defeated them with a Fire-type- a good idea, as it saves you time hunting down new Warriors between fortress captures.
With the battle over, Motonari admits defeat, looking rather glum about it. He praises your worth as a Warlord, asks to be let be in peace (over the objections of his main lieutenant), and requests that you care well for Greenleaf now that it is yours.
You are allowed to take and situate your new Warriors in any of your now-three fortresses, to guard your Kingdoms. Once you’ve set that up, the game provides you a chance to save- and again, ends the month immediately afterwards.