This thread talks about the red eye hatred debuff of Dulgabeth. Apparently, you can't hit him when it's up in order to keep his damage from tiering up.
It will be a pain to avoid hitting him if we use DoTs.
This twitch stream shows a mini kiting Gorkasak, fight starts about 39 minutes in. https://www.twitch.tv/videos/184581346
It shows the whole instance. The last fight is tough.