There is a bug with the tentacles in securing Goldenlight Hallow become invul and unable to be killed. This is not just 1-2 tentacles, this is pretty much all the tentacles, causing the 10 minute Concordia clear achievement to be unobtainable.
I am not sure what cause it, it could be me running too far ahead from Magister after rescuing her. I had this bug 4 times and on the 5 time I run after her and picked Braham instead of Rox.