I actually just did the Mousse earlier tonight, my chars are lvl 36-38, well Vaan is now 39. But the Mousse wasnt too too bad.

Here are some tips for him. Equip your chars with immunities to blindness...or if you want poison, but i dont find the poison to be that bothersome, so i went with blind immunities.

And towards the end of the battle he will start to cast an ice or water spell, cant remember which...and just keep doing it over and over. Kind of annoying, but just make sure heals are kicking in and it should be ok.