{SPOILERS}
It looks like Kamui and Aqua are committing suicide (maybe because they can't choose??)
But at the same time they seem to know what they're doing...
I found severa's to be easier if you had a couple paired up flying units to go ahead and wipe out anything that might get in her way. And both Nah's and Severa's levels become harder if you're trying to get all of the treasure.