How to beat Sphinx, Devourer of the Unworthy, in Valor Mortis

Sphinx, Devourer of the Unworthy, is the Chapter 4 boss in Valor Mortis, found past the Ancient Lair Lantern and best fought at character level 20 or higher. Parrying four or five times fills its posture meter for a head stab worth roughly one-sixth of its HP, and in phase two its face becomes a weak spot that blunderbuss shots can hit for about one-fifth of its HP. Beating it rewards 20,000 catalysts.