Elementary

I never ran this raid during Cataclysm, so I had to search around a bit to find the information for how to complete this achievement first time. Once I got it figured, though, it was extremely simple.

For anyone else like me who isn't familiar with the fight, it is made up of three mandatory phases and 5 individual bosses.

When you first enter the room and clear it of the elementals wandering around, you may enter the first phase with Ignacious and Feludius. Before you pull, position yourself near the centre of the room, preferably around the edge of the purple area on the floor. This is the area you should aim to remain in throughout the fight, as the achievement relies on the final boss spawning in the centre and not moving.

Attack both bosses, distributing the damage as much as possible, as when one of them is defeated both will vanish and the leftover health of both will carry over to the final boss. When you have taken them out, the next two will spawn - Arion and Terrestra - and you can repeat the same process. Once they are both down they will disappear and then all four will reappear and coalesce into the final phase of the encounter, the Elementium Monstrosity. You will be stunned for the few seconds it takes for this to happen, so you need to be in position beforehand.

When the final boss spawns it will create one of the ice pools underneath it. To complete this achievement, do not move it at all! If you were careful to defeat both pairs of bosses as close to simultaneously as possible, this final boss will not have much health, so you should be able to burn it down very swiftly.

Congratulations, the achievement is yours!

-my dear Watson.

The only hard part now is the pulling in P3: the tank needs to be in the middle before the phase starts. Note also that if the tank is exactly on the monstruosity, it will move back and pop 2 pools, so you want to be in front of the boss, like 2-3 yards away.

Then don't move and just nuke it down.