The thing that slows this fight down is his Infernal Soulstorm. It isn't hard, just makes the fight longer.
To get around this:
Wait until he starts to charge "Flames of Ire", this is a 2 part move where he charges it up before releasing it.
He usually casts Infernal Soulstorm when his health reaches a certain % but if he is in the middle of using Flames of Ire, he has to finish using that ability before he casts Infernal Soulstorm.
This is where you use all your cooldowns to kill him.
