Etna was apparently busy killing off the stronger Warslug enemies that dwelled in the swamp, somewhat angered that they couldn't find Zenon. One of her Prinnies remarks that they saw his daughter, and even bought pictures from a talking frog.
Adell, Rozalin and Tink soon appear with Prinny from before who identifies the princess. Rozalin attempts to challenge Etna, even going so far as to take a crack at Etna's lack of endowment. This causes Etna to display her power at that point, uppercutting a Prinny into the sky. When Rozalin takes another crack (calling her a 'flat-chested fornicatress'), Etna uses a Chaos Impact to retaliate, though Adell effortlessly blocks it. The battle starts afterwards.
Etna is level 1000, and unless the player knows exactly what it is they're doing (grinding), the battle cannot be won. Losing the battle continues the story, where Rozalin, possibly facing death, admits she has never even seen her father before. Winning the fight, either through careful planning or plainly outmuscling Etna, earns a special ending where Adell is granted the title of Demon Lord. Tink and Rozalin comment on this, and Adell is not happy.
When the player revisits the map, Etna and her Prinny Squad are replaced by several Mothmen.