Stay Chill

Just solo-ed the achievement on a 582ilvl Fury Belf warrior lvl 90. (specced into Impending Victory for more heals)

7 easy steps to get 7stacks:

1) kill Anshal
2) Jump and stay at Nezir till you gain 3stacks of the debuff
3) Jump and kill Rohash asap
4) Jump back to Nezir and stay there gathering stacks.
5) Wait till Anshal is active again and let him finish Zephyr cast, then jump and kill him.
6) Jump back to Nezir and gather stacks if you are still short and kill as soon as you hit 7 stacks of Wind Chill
7) Enjoy the achievement.
*PS: I lost the 3 stacks during the Rohash encounter, so just continued and gained the remaining stacks at step 4-6.
Still reached 7stacks and downed the Nezir as the last boss*

Easily soloable on whatever difficulty (10, 25, normal, heroic) you want in Legion.

1) Go left, kill Anshal.
2) Go to the middle, stand around and wait for Nezir to give you 3 stacks.
3) Go to the right and kill Rohash.
4) Come back to the middle and get another 3 stacks. You'll probably start to take a little damage here.
5) Go kill Anshal again.
6) Come back to the middle, kill Nezir after he gives you your final stack.
7) Go kill Rohash again.

ez pz

After reading several tips here I found them overly complicated so I'll explain how I managed to do it, as 638 Beastmaster

1) used [spell=120360] to quicly kill Rohash (the boss on the right as you enter, not Anshal like everyone is saying);
2) Jumped to Nezir' plataform, and just auto-atacked him once or twice while I crossed his plataform;
3) Jumped to Anshal and again used all my Barrage
4) jumped again to Nezir plataform, at this time I had 2 stacks, and just waited till I had 7 then I used Barrage again.

At 638 I managed to do this while Rohash and Anshal were still recovering :)