I just killed this fucker last night on my 87 lock, after trying for about 2 weeks for hours each day. I crafted the shadow resist gear from ashetongue rep, tried int flask, stam flask, mastery elixir, vers elixir, int pots, mastery food, vers food to no avail. So I tried to see if there was some kind of gimmick I could exploit/make use of. And I remembered trying to summon a pet (as demo) on the flying disks my first time through and getting a message like "you cannot do that here" or something. So, I got to thinking if the disks are using different logic than standing on the ground, from a programming perspective, maybe it would cause Thyrinar to act differently. There are 2 disks that are close to the edge on Thyrinar's side before you start making your way across, one of which he destroys. I tried to get on the other one on the right (long back at the chasm during the fight) , but could not reach it. I tried the one on the left next attempt, as you have to take a different route on the disks so he does not destroy it. You can reach that one. Thyrinar did not fly over to me though. He just stayed on the platform, but he stopped casting spells. Jumping off the disk plummets you into the chasm, so, after another attempt I placed my demonic circle on the ledge and teleported to it immediately after jumping off the disk. Thyrinar stayed in place and never casted again. He also did not spawn any adds. So, just spent the next 3 or 4 mins DPSing him down.