Solution above pretty much sums it up, but just finishing transferring playstationtrophies.org's solutions over to this site, as true trophies needs proliferation with more solutions.
You need to kill 500 enemies by setting them on fire with the use of Thera's Bane. This trophy is cumulative and you'll most likely have to grind this trophy out after you completed the game. Once you've completed the game, head over to the Combat Arena found in the Treasures options. Now select any enemy type you wish to face in the arena such as the Atlantis Males or Females and incinerate them with the (hold) R1 Button + Square Button buttons. This will take some time to incinerate 500 enemies, but it can be done in about 10-20 minutes.
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-...