I searched this and did not find anything related. I consider this more of a exploit than a guide.
Heigan the Unclean - NPC - World of Warcraft
This boss does 2 things that makes this boss hard :
Eruption - Spell - World of Warcraft<-- With this glitch u wont have to worry about this.
Decrepit Fever - Spell - World of Warcraft <-- This can be dispelled.
Ok i have poested screenshots below so i will explain now:
when your Ranged dps / Healers are in the spot have your melee dps and your tank stand on the platform until phase 2. Once he goes into phase 2 have you melee dps and tank run back to where ur ranged dps and healers are. You will not get hit by the Lava / Poisen If your are in the spot.
ScreenShot Below:
Use This While You can because it makes this fight so much easier and will be fixed soon.