White Mousse

T0 answer the both questions, just use this video, it helps alot:

Http://www.youtube.com/watch?v=s8F2derEZDg

Sorry about the quality but it was the best I could find.

Note: Keep in mind that in the 1st area, where the save crystal is, if you move the switchs, some areas may be filled with water, just turn them all off. About the fight, it's really easy, the best thing you can use is dispell, cause the White Mouse has alot of status effects.

Hope I Helped

If you want to complete the White Mousse hunt, then you'll have to be around level 40 or something pretty high. That's because you have to go through the Barheim Passage which is full of Skull Knights, Skull Mages, Steelings and other stuff. Also, there is an esper down there, the one called Zalera. You have 5 minutes to kill him otherwise it's a battle restart.
At the end of the Barheim Passage, there is a door which leads into the south-western corner of Garamsythe Waterway. The White Mousse mark is in the south-eastern corner. You can't get to it through Lowtown because the pathways to the south-east have been flooded and cannot be crossed. So you have to go through Barheim Passage.

White Mousse is another one of those big jelly-blob-like things, except it's just a pumped-up version. It has two normal ones flanking it as well. Kill the normal ones first because they just get in the way then just bash White Mousse into the ground. Or you can just use espers to have fun.