This achievement is unlocked by defeating 500 Spiders. You can find them throughout Neverwinter, especially in the Defend the Village Skirmish & The Temple of the Spider Dungeon. This achievement unlocked naturally without much effort as I was leveling up to 53. 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.