Kill 15 Sra'thik attackers at the Niuzao Temple within 5 seconds.
There are two ways of earning "Yak Attack":
Now, going by the name of the achievement, it's obvious Blizzard designed the achievement to be completed by the latter means.
Here is how I completed this achievement as a Fire mage, avg. ilvl 518 equipped. I used the following Talents: Ice Barrier, Blazing Speed, and Cauterize. Also, have Ice Block ready.
I hope this helps mages. If anyone needs clarification, just send me a PM.