You simply need to visit the castle in Marche in order to begin Van’s sidequest, where he’ll mention something called the Rainbow Spritnite. Van says it’s in the Archives, so head over there to initiate another scene. Welp, it looks like Van’s childhood friend went into the Archives to retrieve the spritnite. Head inside and venture straight north, into the second area, where another scene will play out. The situation will diffuse and it looks like you had obtained the spritnite, but Van’s friend played a little trick on you. Exit the Archives for another scene, then you will find your next destination, Marsh End. Feel free to set up your party outside of the dungeon, then head inside and make your way to where you fought the boss on your previous visit (don’t forget to save) to encounter a new boss.
BOSS - Giant Toad
Giant Toad is a reskin of the boss you faced in the Holy Cave, only stronger, of course. Soap Breath will hit all of your characters for around 100+ damage, while Tongue Whip hits straight in front of it, so you can avoid that attack by sticking to the sides of the arena. One of the more annoying moves is Sleeping Gas, which is just as it sounds, as it has a chance to put your characters to sleep. The most damaging attack is Frog Rain, where the boss will jump into the air and slam into ground, causing 200+ damage to your entire party (even more if they’re asleep).
One of the first things you will want to do in this battle is inflict slow on the boss, which can be done with something like Lumina’s Earth Bind. Besides that, putting Physical Mar on the boss and keeping it there throughout the battle, preferably with different characters (since each character has their own immunity for ailments on enemies). Apart from the above, you don’t really have much else to worry about, other than the normal “keep your health up” strategy. Keeping your characters with 200+ health is ideal, because of Frog Rain.
The boss goes down and another scene will occur, with you obtaining the Rainbow Spritnite in the end. The sidequest will be over at this point, but you can bring that Rainbow Spritnite to any magic consortium to unlock the Signature skill spritnite for Van.