Rubble and Roll (10 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, they 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 25 Rubble creatures have spawned 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 Normal Difficulty

Easy Achievement at 100, should work for 10 and 25.

1) Start the fight.

2) Don't attack Kologarn.

3) Kill Right Arm without AoEing or hitting Kologarn.

4) Notice that 5 elementals spawned after you killed the right arm, take 'em out, without damaging Kologarn.

5) Kill Left Arm without AoEing or hitting Kologarn.

6) Another 5 elementals spawned! Kill them too.

7) Rinse & repeat a few more times, until you have 25 elementals killed.

8) Once you have the 25, kill Kologarn with whatever health he has left.

See, the main thing is to take out the Right Arm before he grabs you with it, and I always start with right because it's better than starting with left. Enjoy & have fun.

Word of advice, Don't wear your legendary cloak unless you want kolo dead right as you kill his arms for the last time.