Since this is turning into a general Illidan thread i have some information to spare which we learned last week whilest progressing towards our first kill. I didn't read this anywhere before so i think it might help guilds learning the fight.
In phase 3 when he transforms into that huge shadowblast spamming thing (i refuse calling it 'demon form' since he's a demon anyway
) we have hunters laying frost traps and shamans earthbinding totems right below his feet quickly. Three seconds before the inner demon spawns we tell every warlock to chaincast seed of corruption on him (even the tank) and the mages to flamestrike the ground. Usually they are dead within 5 seconds unless 2 or more warlocks get paralyzed. If they get out of seed-range hunters can easily finish them off and melees don't need to risk running through the raid and to flameburst damage to other people.