So I joined a raid group to kill the enemy faction leaders. Since my hearth was down and the group were already in the barrens, I had to make my sweet way across the world. That being said, when the raid group killed Cairne, I was still in the barrens. however, I got the achievement none the less.
However, from what other people said, for cities such as ogrimmar, you need to be in the room when thrall is killed to get the achievement. So I'm not sure if to get the Kill Cairne achievement w/o being in TB is for carine only or some kind of bug. If someone can confirm it would be great.