Rubble and Roll (25 player)

It turns out this achievement functions differently then the tooltip suggests: a total of 25 Rubble creatures must have spawned at the time Kologarn is killed. There is no need to have keep all 25 alive simultaneously, the Rubble creatures can and should be killed as soon as possible.

Killing either arm spawns 5 Rubble creatures, stunnable adds that melee for 4k and cast Stone Nova, an AoE damage spell that also generates a stacking +5% damage modifier.

Killing an arm also reduces the health of Kologarn by roughly 15%.

A killed arm respawns after 15 to 30 seconds, so it is possible to kill an arm multiple times, spawning 5 Rubble creatures each time. These adds should be offtanked and killed as soon as possible, since they can easily kill healers and the stacking debuff makes it unwise to tank them for a long period of time. Killing 5 arms in total generates the required number of Rubble creatures for this achievement, and does enough damage to Kologarn to bring him to a low health percentage.

So in order for the raid to get this achievement, they will have to focus on the arms and adds while doing low damage to Kologarn until the required 25 Rubble creatures have been killed and DPS can focus on Kologarn.

Edit: Don't know since when, but the tooltip has been clarified. Old version: Defeat Kologarn with 25 Rubble creatures alive on Heroic Difficulty

25 Rubbles must be SPAWNED. Doesn't matter if you kill them or not. 5 of them will spawn for each arm killed, so leave the body alone until after you have killed 5 or more arms. Also, he will use Stone Grip even if you're alone. If that happens he will despawn due to not have a target. To avoid this, kill his right arm IMMEDIATELY. Overall, pretty easy achieve to solo for anyone and any class.

Beware if you defeat Kologarn through his arms without actually tapping Kologarn himself. He will die, but you will not get kill credit, and will be unable to loot the chest he drops. A popup window will appear asking if you want to join the instance in progress, because the system did not register you in combat with Kologarn himself at the time that he died due to the only damage he took was via the scripted arm damage.