You'll get this trophy for killing 500 enemies classified as Bandits. I believe enemies referred to as Pirates work as well, as they use almost the same enemy models. However Nashers, the enemies found in the Blacklake District, DO NOT count for this trophy.
The best place to find Bandits is the Blackdagger Ruins. You'll likely even get your 500 kills before finishing the adventure zone questline. Pirates are found in the mid-to-later parts of the Pirates' Skyhold.
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 Bandit Slayer entries.