Let's see... AC 24 with Shield of Faith, 30 HP, Fast Healing 2, Damage Reduction 5/Cold Iron or Good, she can fly and cast Invisibility at will (Among other things).
I've always had bad experiences with quasits in the past, fights against them tend to drag and drag along because PCs can barely damage them and the quasit doesn't have enough damage-dealing power of its own to really compromise or scare the party. I like the Catacombs of Wrath as a whole and think it will be a decent challenge for the PCs but man, I can already predict this fight will be a letdown, the party will run away, rest, come back, try to find some way to beat her, fail again, etc.
I doubt anyone has playtested this part already but what are your thoughts on it?
-Marco