You can obtain this Hunt once you have achieved the Clan Rank of Vanguard. This means that you will likely receive this Hunt way before you are strong enough to attempt it or before it becomes ‘efficient’ to attempt it without having to employ too many work-arounds and strategies.
Defeating this Mark is required in order to obtain the Broken Key which turns into the 
Sluice Gate Key, which allows you to adjust the water levels of the various areas in the Garamsythe Waterway.
Sorbet, the Moogling device operator in front of the Aerodrome at the Rabanastre West Gate.
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).
It can be found in the West Sluice Control area. You will likely need full MP in order to defeat it, so it is recommended that you travel to and find its spawn location and then return there after touching the Save Crystal to refill your MP.
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 setup 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 purchase 
Goggle Masks to prevent Blind (without using up all of your MP). Use Quickenings or Espers if you are having trouble bringing it down.
A video of the battle in the Zodiac Age version is included below:
Return to Sorbet back 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.
This Hunt is normally completed during the intermission period in the Rabanastre following the events at the Stilshrine of Miriam. Click on the link to return to that section of the strategy guide.
