You'll get this trophy for killing 500 enemies classified as Orcs. You'll know they're Orcs because the top line in the enemies name will include the phrase, example: "Orc - Beastmaster". NOTE: The wolves the fight with the Orc Beastmasters are also referred to as Orcs and DO count towards your 500 kills.
The best place to find Orcs is in the Tower District adventure zone. You'll like even hit 500 before finishing the questline because you'll be sent to kill so many.
To check your progress on this trophy first press the Options button > Quests > Journal > press R1 to go to Achievements > Slayer > hit
to show them if they aren't visible and scroll down with R-stick to the Orc Slayer entries.