Quest #089 One Latht Tehth - Swinedimples Academy
Quest Requirements: Defeat 50 enemies on the 10F or lower in any grotto. You can't wear any equipment while doing this
Quest Pre-Requirements: 100 Points spent in the Fisticuff Skill Tree and Side Quest #089 Complete
Reward: Fearsome Fisticuffing and Miracle Moon Ability
This quest is given to you by a man in the southern part of the large gymnasium like building in the west part of Swinedimples Academy L1. In order to accept this quest you'll first need to complete Side Quest #089 which is given to you by Cecil as well. You'll also need 100 Skill Points spent in the Fisticuff Skill Tree.
For this quest you will need to unequip all of the gear of one characters in your party and on the 10F or lower of any grotto battle 50 enemies and win. Obviously the character who is naked is going to be extremely vulnerable so it's recommended that you bring a Paladin with you or someone who is capable of protecting this character each round.
If you're unfamiliar with grottos and how to complete them I would recommend you read my Walkthrough Chapter 9 page as it covers a lot of the Dragon Quest IX's post game content, grottos being one of the things included.