Stay within short to medium range.
It's a mechanic that only activates when you go to far from him and it forces you to stay near him. Just do not go to far.
Thats exactly the opposite of what you want to do if you want to have an easy fight.
Everyone suggesting short medium range definitely has not seen this strategy in the video. Its cheesy but if you wanna win you can do it this way easier than the other methods, because you will never be dealing with his minions if you do it correctly. The teleport is easily telegraphed by the boss icon in the minimap.