This achievement is unlocked by defeating 500 Aberrations. You can find them throughout Neverwinter. Foulspawn creatures also count towards the 500. You can find a good deal of Aberrations in the Aberrant Assault Skirmish. This achievement unlocked naturally without much effort as I was leveling up to 60. 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.