Head back to the Guardian Tree, then prepare for a boss fight.
Boss: Melusine
This boss has 4 forms, but it always starts with the form that is weak against fire. Other forms are weak against ice, thunder, and the last one is immune to all elements.
The boss will randomly switch to other forms after a few turns, which means that you'll have to use different elemental spells to accommodate her weaknesses if you cannot kill her during the initial state when she is weak against fire.
The quickest way to kill her is to change everyone's job to Black Mage (equip Flame Rod) and spam Firaga. It should take only 2 or 3 turns.
After the battle, Lenna joins the party.
Make your way to the ship for another cutscene.
