This is what worked for me, not that it will work for you. 
I had this fight freeze several times, and did NOT want to rebuild my character for this.

Someone mentioned 'damage when he transforms' so - the MOMENT he starts any dialog, run from him (do not damage him)   I have a 12% damage extension on bleed, rend,  etc.  and this let the fight and transformations work and I was able to kill him.  Be warned it makes the fight harder because of the spawns and AOE's.  If you wait he will teleport to you, but freezes you too, your choice.

Hope this works for some of you - its frustrating as (yeah that)