A Dream Within a Dream

Our guild completed this last night.
Through trial and error, I wanted to share what we did to achieve this.
Have /sleep macrod or copy and paste ready because you may need to type it more than once, and time is of the essence.
Have your chosen few ready to slumber in advance.

The start of the fight is not suitable, flowers spawn really fast and will disrupt your sleep within seconds. 30% is not suitable as the speed of flowers spawning increases.
The best time to do this we found was the minute the wardens became active, flowers are not spawning during this phase, so no ticking damage to wake you, just ensure you are not going to get splash damage from the warden (big treeant).
As dream state, you can cross the lines on the floor without taking damage, reach the butterfly as fast as you can. The butterflies are on the edges of the encounter. You need to physically click on the butterfly for it to count. Running over it will not. If you mouse over the butterfly a cog will appear. They are light green in colour, and not the easiest to see but it will be the only thing flapping slowly around. The butterflies do not roam very far, and they are scattered so don't have everyone whose sleeping stand together, spread. If you fail to collect a butterfly, just remember roughly where it is and sleep closer to that spot, this will help you reach the butterfly quicker.

After 5 players have clicked on 5 butterflies. The matriach will spawn, we timed this to just as the wardens died. The matriach it will immediately start lashing out white orbs, so between soaking flowers you need to dodge as the knock back can be unfortunately timed killing you easily. We burnt down the moth fast (it needs to die before the boss does), but there is a bug whereby the moth reappears after another warden phase, do not kill the moth again as you will not be awarded the achievement. Just CC and have no one attack it. It appears that Blizzard have since fixed this bug, so killing the moth the second time will not cause any adverse issues, other than possibly your health pool. At this point just focus on the boss. and voila congratulations :D (I got the staff I wanted too, so win-win)
Goooood luck!

Cannot be soloed. One player can only click one butterfly.

This bugged out for us. We activated the Monarch around the start of the second main phase and killed it. Then we got a second spawn in the third main phase, and killed it again. We did not get credit for the achievement.

EDIT: we re-did this and it re-spawned again, but we didn't kill it this time and got the credit. So if you see it respawn, just ignore it (maybe have a tank lead it away) and kill the boss.