Demogorgon

alright
so here's the reason Demogorgon is hard:
he has 290 base HP, so 680 on LoB
which is 90 (so 180) more than Abazigal's second phase
which is the only phase that really matters
furthermore he regenerates 36 HP every round
this fight lasted around 7-9 rounds (didn't count, feel free to)
so that's like another 290 HP
as the fight starts he spawns two units that are basically like
strong, endgame level, normal enemies
not quite miniboss level
those enemies are like
some of the most tanky enemies in the game
you'll see in the video
furthermore he uses special gazes
one gaze confuses you
one does hold person
one deafens you
he attacks you both with a magical weapon - type attack
and a nonmagical weapon -  type attack
the magical weapon type attack is +5
so absolute immunity doens't negate it
only protection from magical weapons does
which will lead you vulnerable to the normal attacks
he also has 90% magic, 50% cold, 100% electric, and 100% fire resistance
as well as 10% against physical damage
on top of this, he casts Stoneskin once every round
he's level 25
which means that the next 12 (or 13? idk if it rounds up or down) hits against him will be negated
with my highest attacks-per-turn setup I only get 9 attacks per round
alright so after all this
here's why it's actually easy:
none of the stuff I listed above, except for the deafening, is actually a big problem
the enemies he spawns you can just ignore
they attack you with magical weapons
and you have protection from magical weapons up the whole time anyway
you can counteract the confusion gaze with Immunity from Enchantment
you can counteract hold person by having the Flail of the Ages +5 equipped
which gives you free action, part of which means you can't be held
sure he attacks you both magically and nonmagically
but that's why you have your own stoneskin
block the magical attacks with prot from magic weapons, and the physical ones with stoneskin
he's got great resistences, so attack him physically
the Stoneskin part is interesting
there's an item called a Wand of Spellstriking
when used it casts Breach (removes specific protections buffs like Protection from Fire, etc, as well as combat protections, such as Stoneskin) and Pierce Shield (removes protections against spells and reduces magic resistance)
now there's two problems with using the Wand of Spellstriking against him
1) if you use one, that'll be your action for the round
so you won't be casting Stoneskin or Protection from Magical Weapons
which you want to be trying to cast basically every round
because of the 50% spellcasting failure
2) they're p rare, you can only get so many of them per playthrough
both problems have the same solution
there's a spell called Simulacrum
which makes a copy of yourself at 60% of your level, and 60% of your hitpoints
and none of the buffs you have on currently
but it also gets your equipped items
so you can just equip a wand of spellstriking
(I have 11 charges worth of it)
make a Simulacrum
and have the Simulacrum use the wand every time he casts Stoneskin
so how do you prevent the enemies from killing the Simulacrum?
you don't
they don't even go for it
probably because it's a lower level, so it's lower threat
idk
and with that, all you need to do is beat him up
the Black Blade of Disaster (9th level spell that gives you a magical weapon for 18 rounds) is hella fucking strong
add a Planetar to the mix
who's really strong offensively and basically can't die
plus Fire Shield (Blue)
which causes ~5 points of cold damage every time he attacks you
(which is a lot, he attacks you like 8 times a round or something)
and you just beat him down
:)