WoWDB

You earn this achievement by revealing all three fog beasts.  The adds are not targetable, so you don't need to kill them - just expose them.  You do need to reveal all three on the same fight for credit.  Since it will likely take three prolonged colour-beam phases, it is ideal to stack healers for this fight since these phases can be healing intensive, and you will likely have to hold off on DPS anyway to avoid killing him before you've exposed all three.  

The mob spawn points are revealed by little white flourishes (same as the ones you see for the coloured adds).  Crossing the correct two beams over that point will reveal that add.  To determine which are the correct beams for that add, you can use something like NPC scan to know which add is there, but most raids just seem to stack all three beams to be safe so no matter which colour it is, you get the right two on the mob.

Someone with a good eye should mark a white flourish with a raid marker or flare at the start of the phase, and everyone should stack there to reveal it when the beams come out.  If they don't make it in advance, red and blue can always run over to it, but you will have to wait on yellow if they don't make it to the marker before the beams come out and this is unideal.  Once you get your bonus fog beast, finish the phase like normal by moving around the red beam to kill the adds and end the phase.  Then you just need to repeat this process in the next two beam phases.  By the third phase, you'll know what colour remains and should only need to stack two beams to expose him, as Durumu only sends out Fog Beasts that have not yet been exposed (so you won't get a duplicate).

The boss does NOT need to die to earn this achievement, but the any raid members who die early in the fight will not get credit.

In case you misssed colour wheel day in kindergarten:
Crossing Red & Yellow reveals Orange Fog Beast.
Crossing Red & Blue reveals Purple Fog Beast.
Crossing Blue & Yellow reveals Green Fog Beast.