Shattered (10 player)

The Iron Constructs adds can gain the buff Heat by standing in the fire left by Ignis' Scorch.
Once Heat reaches 10 stacks, the Constructs gain the buff Molten, which lasts for 20 seconds.
If within this time frame they are kited to a pool of water they will loose the Molten buff and instead become Brittle for 15 seconds.
A single blow dealing 5000 or more damage to a Brittle Construct will shatter it, causing an explosion. Doing this last step to 2 or more Constructs within a 5 second window will grant this achievement.

The Constructs each give Ignis a stacking Strength of the Creator buff, increasing his damage by +15% per stack. Three of these buffs tends to increase Ignis' damage output to levels above the effective health of your average tank. As a result, the Constructs should usually be killed as soon as possible. For this achievements, two will have to be alive at the same time, putting strain on the tank and healers and limiting the amount of time available to execute this achievement. Note that off-tanks are viable targets for Ignis' Slag Pot ability.

Yes this works. After lightly engaging Ignis the boss, a few Iron Constructs will eventually appear. When Ignis scorches the ground make sure the iron constructs are standing in the fire for 10 seconds. If you select one of the Iron Constructs it will show you a Heat number counting up to 10. Then it switches to a Molten picture. It will even say "Molten" if you hover your mouse over the picture.
Move into the pool of nearby water and the constructs will follow. The picture and word changes to "Brittle" once the Constructs are in the water. Knock out both Iron Constructs within 5 seconds of each. I did so individually rather than AoE. Then clean up the boss. Easy soloable as a 650 Shadow Priest. Then the Shattered Achievement is rewarded.

Soloed this with my 85 DK in 417 item level gear. Its just a basic tank and spank. I did it in blood spec with tanking gear. Pull him to the water and tank him there. Make sure to spread diseases around when the constructs show up. Also got the achievement Stokin' the Furnace (10 player).

Here's the video:
http://www.youtube.com/watch?v=TQsoT5rp8_o