You will fight the same boss from stage 15 and 16.
松葉 (Matsuba, the red hair little girl), 玄爺 (Genjii, the old turtle).
Matsuba now has 26000 HP and Genjii has 15000 HP.
This battle isn't very difficult. Genjii no longer have any magic defense so you can bring in a magic user to damage him. As usual, Matsuba is weak to lightning and Genjii is weak to Light/Dark (i recommend using light since Genjii will cast Light Field magic when his HP gets low).
You can use Byakuren to power down Matsuba to drop her attack to 0. Also, use her dark sphere on Genjii to power down his magic attack by half.
If you defeat one of them, the defeated boss will heal their partner's HP to full. Also when Matsuba HP gets low, she will start casting a spell that blocks magic attack for one turn as well as evade up spell.