@oginam: Wait... you're telling me the inability to lock-on to her is intentional? It really comes off as a bug, but now that you tell me you can light the room up it makes sense. That shit was driving me nuuuuttts in NG+ with those fucking pyromancers and Lucatiel's inability to stay alive when she's not walking herself off cliffs.
I haven't confirmed it but it seemed incredibly obvious to me that her AI tries to "hide in the dark" (basically watched her keep jumping away from me and the 2 player summons I had with me more than she actually attacked us) - so preventing players from staying locked on to her unless she is fairly close makes sense. I would imagine both her AI routines and the lock on range change when you light the room up.