Disarmed (25 player)

Destroy his arms.

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

Be careful with this one.

There has been some change, allthough it may only be with Blizzards parsing of the log files...

We got him to ~28%, then killed both arms - which killed Kologarn, since each destroyed arm deducts 15% of his HP.

We didn't get the achievement and the reply to our ticket was, that one arm was still alive when he died, which is nonsense. I can imagine that the logfiles are parsed diffrently maybe in current patches, than they were in 3.x, so this was the way to go but is not anymore.

We will try to get him to 40%, kill both arms then kill him, which will hopefully do the trick...

The trick to this achievement is that if both arms are destroyed Kologarn will begin to cast Stone Shout on the entire raid. Thus you must ensure that Kologarn dies soon after the second arm is destroyed or just stack enough raid healing to live through it.