Excalibur is the most powerful end game weapon for Melrose. Out of all the ultimate weapons, hers is the easiest to obtain.
You can start the quest associated with Excalibur by talking to one of the creatures in Meylin Underwater Town. This is the area that's accessible only by taking the Pirate Ship from Pirate Cove. You'll come across the makeshift underwater town (an area with trees and mermaid looking creatures). Most of the creatures are an orange/yellow color, but the creature you'll want to speak to is a dark purple color (who's name is also Meylin).
If you've completed the "Collect 3 Flower Powders" quest in Lekora, she will note the smell of the powder on you. She will ask you to get one of the yellow flowers from the Fureta Jungle. There are 3 yellow flowers in all, and any one of them can be interacted with to get the necessary item. If you've activated the Memory Teleportal in Fureta Jungle, this becomes even easier because there's a flower just north of the teleportal. The flowers do stand out and are pretty easy to find regardless.
After gathering a flower, you'll have to go back to Meylin Underwater Town from Pirate Cove. Talk to her again, and she will exchange the flower for the Excalibur.
I have never tried to speak to Meylin without first completing the first flower quest in Lekora. However, completing that quest may be necessary first. If she does not talk to you in order to activate the side quest, complete the side quest in Lekora first.