A Race Against Slime

So Ariok survived his encounter with Kilrogg's forces, and was only subdued, not killed.

The Gamon precedent implies the results of "hard-mode" achievements are canon, and this is backed up even more by the fact this achievement grants Ariok as a Follower.

Awarded if you manage to kill [npc=90378] in under 5 mins, 15 seconds on Normal, Heroic or Mythic, though naturally normal is recommended. You can allegedly extend the timer by having someone with [spell=187089] stand nearby Ariok, reducing the damage he takes. If you can kill Kilrogg on heroic, this achievement is very doable on normal as a reference.

Curiously, the follower interface lists this achievement's name as 'Between Ariok and a hard place' which was actually the placeholder name for the achievement during PTR.

They literally just used this pun for the Spires of Arak storyline achievement...

(Note: PTR Name for the achiev is Between Ariok and a Hard Place in case this changes later)