Hunt 06: Lost in the Pudding
White Mousse - Rank V
You can obtain this Hunt once you have achieved the Clan Rank of “Vanguard.” However, you’ll likely receive this Hunt way before you are strong enough to attempt it, or, to be more specific, before it becomes ‘efficient’ to try it without having to employ too many work-arounds and strategies.
Defeating this Mark is required to obtain the Broken Key, which turns into the 
Sluice Gate Key, allowing you to adjust the water levels in the various areas of the Garamsythe Waterway.
Petitioner
Sorbet, the Moogling device operator in front of the Aerodrome at the Rabanastre West Gate.
Location
You can find White Mousse back down in the Garamsythe Waterway, which has changed quite a bit since your first visit (it changes after you finish the Tomb of King Raithwall).
You can find it in the West Sluice Control area. You’ll likely need full MP to defeat it, so you should travel in and find its spawn location and then return there after touching the Save Crystal to refill your MP.
Hunt Battle
Level: 35
HP: 69,469
Steal: 
Potion, 
Echo Herbs, Hi-Potion
White Mousse has a heavy artillery of attacks. You should cast both 
Blind and 
Berserk on it if possible, but this will make its attacks significantly stronger, which means you need to be prepared to deal with that onslaught. Make sure that you have a proper tank setup with 
Decoy cast on that party member. Your tank should have Bubble Magick active during the fight or have a 
Bubble Belt equipped.
One of your party members should have an 
Esuna Gambit setup, as White Mousse can attack with Blind and Slow status effects. Due to the amount of damage done, you should also consider having more than one party member set up to assist with healing.
In the original PlayStation 2 version of the game, after the battle against Judge Bergan, and once you have completed the Stilshrine of Miriam, you can also purchase Viking Coats to reduce the amount of damage done by Waterga, and you can buy 
Goggle Masks to prevent Blind (without using up all of your MP). Use Quickenings or Espers if you are having trouble bringing it down.
You can find a video of this battle in the Zodiac Age version below:
Return to Sorbet up in Rabanastre proper to receive your reward, which includes 2,800 gil. Sorbet will take the Broken Key to get it fixed. He will decide he doesn’t need it, though, and will give you another key, the Sluice Gate Key, as a reward for assisting him.
You should complete this Hunt during the intermission period in Rabanastre following the events at the Stilshrine of Miriam.