Celestial Challenge

You don't need the quest to get this achievement , you only need to kill it with a raid if someone has the quest.

As of 5.4.2, you don't even need to be in a raid — you can join an ongoing fight started by someone else, and the kill will be counted for this achievement (you also will get the loot if this is your first kill in the week).

I noticed "The Four Celestials" as a World Boss option using the World Boss Status add-on for tracking Sha of Anger, Nalak etc. (Good simple add-on btw).

Obviously in Pandaria this was probably a lot bigger of a deal but now its but a simple weekly drop for some loot and some currency. You also get 500 rep with The Black Prince.

Despite what it indicates - you only need to do one celestial and don't get anything for doing the others.

The real trial is listening to the ramblings of the celestials each time.