This achievement is unlocked by defeating 50 Giants. Giants are a little more scarce in Neverwinter, but they can find Frost Giants in Icespire Peak and Fire Giants in Mount Hotenow for the kills. This achievement unlocked naturally without much effort as I was leveling up to 64. 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.