Quest #081 Mallet's Mallet - Swinedimples Academy
Quest Requirements: Read Mallet Monthly in the 1F Library; Go to Plumbed Depths in Gleeba and fight Admirer enemies until you get Wobbly Jelly
Quest Pre-Requirements: 30 Skill Points in the Hammer Skill Tree
Reward: Terra Firmer
This quest is given to you by a girl in one of the classrooms in the middle of the area on the 2F of Swinedimples Academy. In order for her to offer you this quest you'll first need at minimum 30 Points spent in the Hammer Skill Tree.
For this quest, the first thing you will need to do is go to 1F of the Swinedimples Academy and read Mallet Monthly in the library. A screen shot of where you can find this book is shown below. Once you've done this you'll want to Zoom over to Gleeba and enter the Plumbed Depths dungeon.
If you've forgotten where the Plumbed Depths dungeon is, it's located in the royal bath portion of the Mirage Mahal (1F). In the northwestern corner of the royal bath there's a small well which you can go into that leads to the Plumbed Depths.
An Admirer enemy (screenshot below) is who you're looking for in the Plumbed Depths. They're most commonly found on B1 of the dungeon in my experience and they'll drop the Wobbly Jelly that we need for this quest. As with most quests in this game, the drop rate for Wobbly Jelly seemed to be pretty low, I would estimate 5 - 10%.