Updated 05-29-2016 02:25 PM
To unlock this achievement you simply need to kill 500 Orcs in the game. There is no real need to grind out the kills as this achievement unlocked naturally for me as I was completing quests and leveled up to 65. If you do want to finish this sooner, you can always repeat The Clock Tower Dungeon located in the Queues. You can also check your status by bringing up the menu, tab over to Quests, access the journal menu, tab once more to achievements, and scroll down to Slayer.