The giants in TOV are not updating the slayer achievement " Might They Be Giants". Initially I thought it was because they were undead, but the undead kobolds in Velks are updating the achievement "Puttin' On The Dog" and "World Warrens Three"
This is a known issue. Some of the new races need to be re-categorized to work properly with slayer achievements.
Likewise, Sharks in CoV's Cobalt Scar are not updating Swordfishmermaid. I think many of the mobs in CoV aren't updating slayer achievements correctly