It took many hours of re-goes, but I finally killed Demogorgon in SOA.
In the past I have given up & just read the scroll.
The strategys from around the net say to use higher level spells than I have, and +5 weapons,
But in SOA the only +5's I have is Crom Faeyr & Keryvian (a Bastard sword I have 1 star in)
Tashia has the Sling of Everard +5, but is flat out spellcasting.
I did it on normal hardness with level 17 to 19 characters.
The people who claim you can stay up the top & take care of his summond creatures 1st
must have been on easy setting.
Because on normal setting he summonds 6 Glabrezu's at a time (if he gets more than 12 against you, you have no hope) & they never end.
Plus contary to what they say, as you can see in the screenshot, Demogorgon is perfectly willing to follow you up the stairs himself.
You just have to hit him hard with everything right at the start, and be Lucky.
The longer it goes on, the less chance you have.
Setting the game to "Pause on spellcasting" is the best tip I read.

My party had just come back from their extended voyage (start of ch. 6) and had gotten to level 20-22 in the caves (most had just gotten their first or second HLA). I had gotten down to the Final Seal before I ended Ch. 3, but the final guardians were pretty rough, so I waited.

Conclusion: pause-on-spellcasting is darn-near-broken when you have a sorcerer cast Improved Alacrity while wearing the Robe of Vecna and Amulet of Power. FYI, Improved Alacrity is a HLA spell for mages that reduces cast time of spells and, more importantly, lets you bypass the "one spell per round" rule for two or three rounds. After buffing up and summoning a couple distractions (One deva from the cleric, plus a couple spiders and skeletons), my sorcerer popped Improved Alacrity, threw Improved Haste on all of my fighters, and then waded in and unloaded over 20 other offensive spells in two rounds. Needless to say, the Demogorgon was not having a fun time of things by the time my front-liners got to him and started wailing away with 8-10 attacks per round.

So, yeah, congrats on taking him out at only L17-19. Must have been pretty nasty. Also, I'm somewhat curious why you got a Glabrezu swarm. When I went against him, he gated in a pair of Marilliths. I guess the summons must adjust to your level.

I got 2 Marilliths to start with & then Glabrezu after I'd killed them.

It was the most reloads of any battle in the game.

I found without "pause on spellcasting", I was wasting spells by interupting them with the next one.