Ok this is not actualy an ordinary guide to this boss, it's a trick I found at that boss which allows u to go trough phase 2 without "dancing" (I hope most of u know what this means at this boss).
->So maybe all of u know that when phase 2 comes the boss goes on his platform and starts casting his poison crap arround. Usualy, when it does so everyone starts moving arround to avoid the explosion. Using this spot I found, u don't need to move, everyone just stays on top of each other and the ranged dps can start nuking the boss without receiving any dmg. (this works on 25 man also)
-> Using this u can easy complete the achievements "The Safety Dance" and "Heroic: The Safety Dance" which ask u to defeat Heigan the Unclean without anyone dieing on normal and heroic difficulty.
-> Ok I tryed to make some screenshots to show u the exact place where u have to stay, I hope they are good:
Ok I hope this is enough for u to understand where u have to stay, I did this 3 times in naxx, once in 25 man and 2 times in 10 man and I assure u it works...well atleast untill blizzard repairs it :P