This ending requires you to fight Lavos immediately after rescuing Marle and returning to the present. This happens in the story line after you rescue the queen in 600 AD, which is consequently your first in game visit to that era after the millennium fair mishap. Once you return to the present Lucca will leave your party, effectively making you fight against Lavos with just yourself and Marle. Much like the Dream Team ending you will need to use the gate located on the right telepod to arrive at Lavos.
Once you beat him you arrive back at the millennium fair except everyone has a frog croaking issue with their voices. Once Marle gets scared enough everyone heads back to the castle to find out what is wrong. At this time they show a slide show where frog marries one of Marle’s decendents causing the current issues.