You Said Crossing the Streams Was Bad

References aside, the achievement comes from revealing all three beasts.

The Orange Fog Beast is revealed by crossing the Red beam and Yellow beam.
The Purple Fog Beast is revealed by crossing the Red beam and the Blue beam.
The Green Fog Beast is revealed by crossing the Blue beam and the Yellow beam.

Good luck and happy achievement hunting!

Here is how to SOLO the "You Said Crossing the Streams Was Bad" Achievement (as of Patch 7.1.5).

This method can be completed by all classes, however it will be easier for tank classes due to incoming damage.

Strongly Recommended: As mentioned by Matt453 above, download the Mists of Pandaria version of Deadly Boss Mods (DBM). There are a lot of boss mechanics in this solo achievement, and this addon is extremely helpful.

This achievement requires you expose the three "mini" fogs. The boss has three phases, and only one "mini" fog can appear in the second of these phases. Therefore, you will have to experience these three phases... three times.

First off, you need to know WHAT to avoid.

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• "Force of Will" is an AoE purple cone that will appear on the platform. This will knock you off the platform and instantly kill you, no matter what level you are. To my knowledge, this appears in all phases.
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• "Disintegration Beam" is an ability that is in the third phase. A beam will pop out of the boss' eye... don't run into it, as you will instantly die.
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• DO NOT DAMAGE BOSS - as others have pointed out in comments, do little to no damage to the boss until you get the achievement. It appears that this decreases how often "Life Drain" is cast.
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Progression of the fight:

Phase 1:

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• Start the fight and keep to the outer edge of the platform.
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• Avoid "Force of Will" whenever it appears. This occurs in all phases.
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Phase 2:

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• MOST IMPORTANT - When "Light Spectrum" comes up... YOU MUST see where the white disc/cloud appears. Failing this means you most likely fail getting the achievement.
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• Run as fast as you can to the location of this white disc/cloud.
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• Once beams come out from boss... wait a few seconds.
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• Observe the "mini" fog that will appear. It cannot be targeted, so don't bother.
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• Now kill the three Crimson Fogs as quickly as possible by running around the platform. Killing all three ends the second phase. This is where DPS specs need to be fast in killing all three.
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Phase 3:

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• Avoid "Disintegration Beam" as it will kill instantly.
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Once all three phases have passed... repeat two more times to reveal all of the "mini" fogs.

- For this achievement you have to stay through three color phases.
- Combining two beams above the 'white' mob will make it get the color you need.
- You should have everyone look out for the small white cloud when the color phase begins and make sure everyone(!) stands on this location with all three beams when the real beams spawn. This way you can ensure that you will get the correct colors, becaus otherwise it seems buggy when only combining two ones.
- It seems as if the boss will correctly spawn the missing colored adds when standing with all beams on the white add.
- You do not have to kill the boss to get the achievement.