Seeds of Doubt

Nobody mentioned yet, but the Seeds of Doubt are four clickable crystals that are present in the four corners of the room, but only during the add phases.

I managed to solo this after figuring it out during the fight and not telling the group.. stood in one corner, stop DPS so we get a third add phase just before the boss dies, then Sprint like mad to get all 4 (using healing pot / self-healing to stay alive).

Didn't risk Cloak of Shadows - would be horrible if it removed the debuff! I will test this next time.

Here is a quick and dirty Warlords of Draenor (and beyond) guide to this achievement! I soloed dungeon this as an elemental shaman, and essentially any DPS class and some healing classes can easily do the same at level 100.

I had no problem doing both Cleaning Up and Seeds of Doubt at the same time, with plenty of time to spare.

First, make sure that you get at least 1 Figment of Doubt phase when fighting the [npc=56439]. In this phase, the Sha will create a "figment" of each player in the encounter, which is essentially a copy that attacks the corresponding player. Killing this figment will end the Figment of Doubt phase. At level 100, it is possible to kill the boss fast enough that one of these phases never materializes! What I recommend is bringing the boss down to about 10-15% and waiting for the Figment of Doubt phase.

There are 4 purple crystals that spawn at each of the corners of the room during the Figments of Doubt phase. The crystals are floating above the ground, so you need to stand beneath them to be able to click them. Each crystal gives you a stack of the debuff [spell=123916], and so clicking all four gives you 4 stacks (hence 100% increased damage taken). Clicking the crystals itself will do some damage, so be vigilant of that and heal up as necessary, or use damage mitigation to ensure you don't get too low. Also, do not use any ability that would clear the debuff, as you need to have 4 stacks when you kill the [npc=56439]. The debuff lasts 1 minute.

It's important that you don't end the phase early (i.e. kill your figment early). Avoid anything that does passive damage, including pets, so that your figment stays alive and you have time to click all four crystals.

If something goes wrong, and you can't get 4 stacks that phase, simply kill your figment and wait for the next Figment of Doubt phase. Otherwise, you can end the phase with your 4 stacks and proceed to finish the boss off!

TL;DR version:

1. 
   
2. DPS the boss low and wait for Figment of Doubt phase
3. 
   
4. Click all 4 crystals at the corners of the room
5. 
   
6. Kill your figment, then kill the boss
7. 
   

We made a video tutorial on how to complete this achievement, you can view it here:

http://www.youtube.com/watch?v=_hf3H1pktsc

The easiest way to do this is to get the boss to extremely low HP (less than 10%). Have one person assigned to click the crystals, and make sure when the add phase occurs that his add is killed first, so he can run around without risking death. Click all four crystals, and then make sure your party kills any remaining adds before the phase ends, or the Sha will heal a lot.

Each crystal puts a debuff on your raid and does damage. The debuff increases damage taken by 25% per stack, so you take 100% extra damage with 4 stacks.