Where is she located? I've been wandering all over the blackpits, and I'm not sure where to even find her. Apparently she's needed to advance a quest for Fane.
There's a cave at the far end of the Blackpits area. It's easy to miss. This encounter is all the way inside.
My team were at the end of level 13 when I got here. This boss wiped the floor with us. The real problem for me here was, she got to go first, and with 8 APs, she could summon 2 wolves, cast Rain, then Chain Lightning, then Flight into middle of team, then Superconductor. Anyone alive after all this would be stunned and we never got any chance at all to begin with.
I had to reload and mess around with my team's stats until Sebille had higher Initiative than her. That alone was enough to turn things around. The key here is to not let her blast full combo at you. What I did was to throw a smoke grenade at the team and teleported her to the back, so in her first turn she simply spammed a bunch of potions. After that it was quite simple. The wolves have very weak magic armor so I was able to charm all the 4 in the middle easily. Like someone already said, I used the Mind Maggot Grenade on these 4, and it was actually quite easy after that. The boss' not that tough against physical damage/cc's. I retried the fight a couple times, and the key really was to just be able to go first. Charm works wonder here, but stuff like Medusa Head works nicely too.