This takes place in the 3rd area called 'Chapel of Revenants' which is a boss room featuring the boss 'Gwen, the Matron'.
This unlocked for me when she reached 1/3rd health after repeatedly luring her to slam her hand down on a platform, leaving it there to be attacked whilst she stabs down with the other hand.
Her hand was severed off at this point and she simply alternated between a laser attack and then slamming the knife down to be stuck in the platform for then easy hits to finish her off.
Here's an image of the simple platform layout I used. I had the four set up like that and then would stand on the bottom right, wait for her to attack and then move left and up, ready to attack when the hand was in place.