This is the fight that will screw new raiders over...seriously. But once you know how to do it it is easy. Before I type anything else, here is the diagram (ty wowwiki) of where the safe spots are;
(Ignore anything to the right of the number 4 on that diagram)
ABILITIES:
Heigan hits for nothing, really. 85% of the time your tank's back is facing the boss. This fight is all a test of raid coordination, and believe me - it's a test.
He does a silence to anyone near to him.
He makes slime come from the floor and kill you.
PHASE 1:
On that diagram, that red block is where Heigan is to start with. The Main Tank stands where the blue dot is in zone 1 (stay with me) and pulls Heigan.
As soon as heigan is pulled all ranged DPS and healers run up to where Heigan was (his platform) you do not need to worry about the slime here).
The tank must stay in Safe Zone 1, roughly the distance from the platform that the blue line on the diagram shows. As soon as he sees slime shoot up from the floor he RUNS (just runs with his back to heigan) to safe zone 2 and then he STOPS.
He will soon see slime come up again, he then runs to Safe Zone 3 and STOPS. Then he will see slime come up a third time and he runs to safe zone 4.
When he is in Safe Zone 4 and sees slime come up he runs to safe zone 3 - then to zone 2 and then to zone 1.
The pattern (The Heigan Dance) of zones is this;
1,2,3,4,3,2,1,2,3,4,3,2,1... etc etc.
Keep this going until Phase 2.
PHASE 2:
He will teleport back to his platform and stop attacking people. Everyone that was on his platform now runs to Safe Zone 1 and does the dance (it's faster this time round).
EVERYONE will be doing the dance at this point. Remember 1,2,3,4,3,2,1...
Then Heigan will rush down from his platform and go for the main tank. At this point everyone who is ranged or a healer goes back up onto the platform and Phase 1 repeats. HOWEVER!!! No matter hwere you are when he runs back down you go to Safe Zone 1. The dance always starts at 1. ALWAYS.
Oh and even if everyone but the tank and 1 healer dies, the fight is still doable.