Illidan's fight was more technical than Kil'jaeden. The video was much more technical to make too ^^. I thus hope you like it.
Prismatic Elixir, Drums of Forgotten Kings, and especially Void Star Talisman, was important to survive the second phase. Fames of Azzinoth inflict many damage and the VoidWalker survives with difficulty without these objects. It's recommended to prolong P1 for wait CDs (DemonSoul, SoulShatter ect..)
This is a long battle, it is always necessary to keep an eye on the life of the Felguard, and use the Metamophosis as an armor CD for this demon.
The rest was not a problem, used the Glyph of ShadowFlame for parasits, manage aggro, and take his time.
Musics by Two Step From Hell :
Call Of Arms
Power Of Darkness