Put Julienne in your active party and travel to Royburg, then speak with Cornelius inside of the town. You’ll learn of the tale of a blacksmith who stole a Spritnite from the royal family. To find out the name of the blacksmith, you decide to go to the site of the former capital, which is the Last Lands. Head to the first area and take a right at the first intersection, where a scene will occur by the second house.
Once you find out the blacksmith’s name, you can go to Archimell Ruins; you will want to use the front entrance of the dungeon. What you are looking for is one of the teleporters that has a red light, which you might have come across whenever you went through the place earlier. It is located on the right side of the second floor, after using the teleporter to get to a lower area. Once you find it, approach it for a scene, then it will become usable, so take it to a new area and prepare for a boss fight.
Note: You do not have to use Julienne in the following battle.
BOSS - Grinche
This boss fight is very similar to the one against Timeslave in the main story, with it using a lot of the same moves. The only difference is that Grinche is a lot more powerful. Missile Punch is an attack that will hit a single character, as is Laser Beam, although the latter might deal a little more damage. Grinche also has access to Slow II, which will hit everyone, so you can either get rid of it with Panaceas or with Endir’s Aura with Momentum.
At around 50% health or so, Grinche will use Rampage, which will make his attacks stronger and even change a few of them. Missile Punch becomes Twin Missile Punch, which will hit two characters instead of one. Also, Slow II is turned into Stop II, becoming an annoying spell if it manages to stick to everyone, as that could prove to be fatal. Grinche can also use Blowback, which was the death move Timeslave used, but Grinche can do it any time after using Rampage. Blowback is definitely the strongest move in its arsenal, so you will want to heal up after it uses it.
Upon defeating Grinche, there will be another short scene that plays out, with you obtaining the Eclipse Spritnite for Julienne.