This achievement is unlocked by defeating 500 Trolls. You can find them throughout Neverwinter, especially in Icespire Peak, Drowning Shores, and The Frozen Heart dungeon. This achievement unlocked naturally without much effort as I was leveling up to 70. 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.