Disarmed (10 player)

Kologarn yells,

"Only a flesh wound!"

A good way of completing this Achievement is to continually kill the right arm, get the left arm low while the right arm is down and when Kologarn is below 30% hp (he loses ~15% hp per arm killed) you kill both arms within 12 seconds, also destroying Kologarn. Note that he must not be below 15% hp or he will die after the first arm dies and you will not get the Achievement.

The benefit of this is that you won't have to deal with the Rubble spawning when the two arms are killed, which can be pretty dangerous.

Also, currently the achievement seems to be working differently than the Tooltip suggests, you either have to kill both arms in 12 seconds and then kill Kologarn without killing another Arm or you have to kill Kologarn within the 12 seconds as well (haven't tested enough to know for sure which one it is).
If you kill both arms within 12 seconds at the start and then proceed as normal, killing Arms inbetween, you will currently not get this Achievement, this is because the 12 second timer is triggered every time you kill an arm, failing the Achievement if you don't kill the second arm within the time frame, even if you already killed two in 12 seconds before (I am not 100% sure about this, a confirmation would be appreciated).

Destroy his arms.

He will then, quite literally, face-roll the raid.

Quote Daftness