However you’re planning on handling it, once your first turn ends, it moves on to Motochika. He likely doesn’t have any move to make, unless you really rushed things and already have someone across the channel or at least a floodgate closed. Whether or not he actually does anything, though, he starts proclaiming something ominous- he talks of an oppressive and powerful black wave in the North, which he predicts will come crashing Southward. He is very sure that he won’t be ‘swept away’ by it, that he’ll continue to fight- an assertion that he might be able to bear out with his channels in place to keep him somewhat secure.
He begins his second turn by noting that you also intend to resist- and complimenting you for it. It is now that he makes his move, even if you haven’t crossed the channel yet. His lieutenant, Morichika, also pipes in, declaring that it’s his time to prove his worth as a Warrior, one of those gifted with the ability to communicate with Pokemon.
If you’re lucky, Motochika’s second turn closes out with one of his Warriors closing the upper floodgate, freeing you to begin your attack.
Motochika is a very strangely polite opponent, actually complimenting you on your first successful knockout of the fight.
Either way, the fight itself is pretty straightforwards once you get around the water channels, and if you’ve managed to get some more grass-types on your side it should be over rather quickly.
Once you’ve one, Motochika admits his defeat rather easily, going on to speak of how his Pokemon are never going to leave him and that he is still going to ‘fight against the tides of fate’. His resolution to deal with this mysterious force is rather impressive, and begs the question if this is something you ought to be worried about. Also, it’s a bit curious that he knows of this force and you don’t, with his being the only fortress further South and more out of the way than your home of Aurora.
With Motochika defeated, you are free to recruit whomever you qualify to from among his forces. If you can, snag all three- while they aren’t perfectly matched with their Pokemon, they’re all possessed of useful skills and their initial Pokemon are very strong early forces- having the ranged attacks helps a lot too.