Talk to Degga at (H-9). (You will see him on your right, toward the end of the entry corridor.) He will apply an ointment that will restrict your level to 20.
Note that, once the level restriction is applied, only others with the same level restriction will be able to cure or buff you.
You will lose the level restriction if you are KO'd. It may be reapplied at any time by speaking to Degga again once raised. (Take note that if you return to your Home Point after being KO'd, or otherwise leave the area in any way, you will lose any progress made in the quest since talking to Degga.)
At the very first 4-way intersection (H-9), go down the West corridor (turn left).
Go down the West corridor and turn North at (G-9).
Continue North at (G-9) and do not turn down any passage but ones leading North until you arrive at (G-6).
Camp on the Western side of (G-6) and clear out the monsters here before turning to the East corridor.
Clear the way to the East. You will find the ??? on a chest further East on the middle of the deck at (H-6). Clear the area around this chest.
You must have the level restriction applied to spawn and engage these NMs.
By default, the Puddings will spawn with sound aggro toward the party member who spawned them. (If Sneak or Silent Oil is applied, the spawner will still be aggroed. Clicking the ??? removes any Sneak effect.)
The Puddings are Slime-type NMs, As such, they are weak against magic, but highly resistant to physical attack. As they gain TP, they will use a strong version of Fluid Spread, the Slime AOE attack, that will cause critical damage to anyone in range. This is a common cause of wipes.
You must have the Level Restriction applied to obtain the Key Item.
Should you change areas during or after the fight, but before obtaining the Key Item, clicking on the ??? will only cause the Puddings to respawn. They will have to be defeated again for you to obtain the Key Item.
Return to Degga to have your ointment/level restriction removed. It may also be removed through logging out, then logging back in.