Story related, can’t be missed. You meet him early in the game but do not fight him until 3/4ths the way through the game, don't worry you've missed it.
*** Spoiler - click to reveal *** Lanaeus is the guard in the volcano in the beginning of the game in The Caldera. As you approach Lanaeus, he'll shut the gate on you. Climb the wall over to the left, and kill the Scylla Brood (the spider-like monsters) that attack you. Drop down onto the lower platform and use your Arms of Sparta to kill him from a distance by using the Spear Assault move with the (hold) R1 Button + Square Button buttons. Once you've killed him to lower the gate your 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-...