Story related, can’t be missed.
*** Spoiler - click to reveal ***This is during the Temple of Ares. A brief cut-scene will happen where you'll see Kratos' younger self pop out of the mirror to kill the present Kratos. Fend off your younger reflection self by completing the quick time events and throwing him into the mirror by simply using the Left Stick analog stick to aim your inner self into each section of the mirror by pressing the Left Stick button up, down, left and right. Each section glows blue and will crack when it's hit. Once you've thrown him into each section of the mirror, Kratos will charge him and will crush him into the mirror by you repeatedly pressing the Circle Button button. After you crush your inner self, the trophy will unlock.
All credit goes to orginal authors at playstationtrophies.org, and this is merely updating truetrophy fans with these guides.
http://www.playstationtrophies.org/game/god-of-war-ghost-of-...