Some information a friend and I found in another thread that is useful to this fight, others are having success with it, so I will try my hand at it and get back here if it works, and will post any additional information I learn while fighting him again. Hoping this works.
The point of the gateway seems to be to get away from where you were beforehand and is also helpful in one of the later phases when he puts out mass fire on the ground. His adds are more powerful than your own demons, so they hold their own against him better than ours.
The scaling is only two levels over your current level, but with the current difference between one level and the next in terms of health bar and power/strength output between the levels, two levels above our own is actually quite excessive when you actually look at the difference in those two statistics from one level to the next. One level up would be enough to keep us from subjugating him and still make him painful to fight, but it would just feel more solo-able.