The progression of Asellus' scenario is, instead, to wander around until the assassins that Ciato sent after you show up.
WaterSage opened with an Oscillation attack that KOed both Lute and Rouge, but other than that I had no problems.
GreenSage (legalize it!) doesn't have many powerful attacks, but likes to use status ailments. The big problem with this battle is that GreenSage has a fear aura, which shows up as a few semitransparent skulls floating around him. If any of my party hits GreenSage with an attack that makes physical contact, the fear aura counters by confusing them. Asellus uses some sword techniques she learned that don't make contact, like WheelSlash and 2GaleSlash; Rouge and White Rose use magic; and Lute got a gun for this fight.
After the GreenSage is defeated, a cutscene:
Lion: A half mystic Asellus and our White Rose... I am afraid Ciato won't be good enough.
Orlouge: Are you going?
Lion: I exist only to serve you, Lord. Besides, White Rose, she's my sister princess. I don't understand why she's disobeying you.
Orlouge: No one's stopping you. Go, Lion.
Lion catches up with us in the Yorkland swamp. Usually those lizards are hiding with only their eyes showing, but Lion's cutscene keeps them from concealing themselves.
White Rose: They say you're the bravest princess in Facinaturu.
Lion: Princess White Rose... I've heard of you as well. They say you're the kindest princess in the region. Can your kindness stop a sword?
Asellus: I am the one who'll fight you!
Lion: Whatever! The only man who ever beat me was Lord Orlouge himself. Get ready!!
Princess Lion attacks with a combination of sword techniques and magic. She's the toughest boss yet, but uh...
She's still nothing compared to the DevilSquid battle.
Lion: Lady Asellus, take care of my sister.
White Rose: Wait. Lord Orlouge will punish you if you let us escape.
Lion: Don't worry about me. For you, I'd gladly accept any punishment.
End of chapt-- uh... Lion? Aren't you cold dressed like that?