Where can I get Regen Vial for the sick girl? Thanks!
Regen Vials are dropped or stolen from Goop/Ooze enemies in the abandoned library and volcano fortress respectively. They can be purchased at the item shop after you do a quest to upgrade their inventory, though this requires a boat. They can also be crafted at the alchemy shop in the witch tower if you find the Healer Recipes, which are in a chest on the western bluff. It's also valid to put the quest on hold until later, as there are some chests in higher level areas with them.
The jelly fish i way more weak than those places, i tink that the vial is more dificult that the quest's boss, that is kind of anticlimatic. I guess that we could have one of theses early in the game. I just abandoned the quest and when i finished all others boat side quests i remembered that girl and back to heal her.