Does anyone have some tips or strategies to kill this beast? I've spent the last hour trying to kill this thing and no luck. This is by far the hardest battle I've came across in this game!

Hmm, try getting potions of magic resistance, and stuff like that. Cast all wonder spells (Haste, chant, etc) run in, and kill all the tutor looking people first. When they all die, go for the beast himself.

The biggest problem with killing him is that he resurrects himslef in the bodies of the cultists. When you go down the stairs have your mage cast otilukes sphere on the demon. He'll be cut off from the fight for a while and be unable to move(you might have to try a few times coz he makes a save)then kill all the cultists so he can't resurrect himself. Then when the spell wears off start hacking him up. This way you'll only have to kill him once.

Spoiler

Here is my method which seems to work well. I invis my whole party then go down the stairs and get to the corner in the NE From there I use range weapons and pick off one by one the npcs standing around saving the high pristess for last. Then I reinvis to Force attack her with again range weapons all except a mage with a wand of Paralyzation who stands ready but does not fire it yet. As soon the rest fo the party fires the talk wil start before the pristess is actually hit. When it ends the Demon appears and i hit him with the wand right away. IF your attacks went well on the pristess the one volloy does her in wend she falls I switch everyone to him. I keep hit tin ghim with the wand till he faisl his save. So far it has only taken me twice to get it to stick on him so I do nto think he has a high resistance A secon dmethod I tried once that worked also was all the same except for when I went to kil the pristess I instead also fired a kil cloud and a web to just in front fo her so it woudl get the demon when he popped in

let me tell you this, with 1 spell this winged fairy is one of the easiest fights in the game. It's called Ortiluke's Resilient Sphere. You cast this bad boy on him as soon as he appears, now you can't attack him while he is imprisoned by the Sphere but you can take a stroll around the room and annihalate the rest of the cult like nothing. Once you have crushed down all the members of the cult, simply cast dispell magic on Aec, and then crush him, make sure to have some dispel magics memorized in case he hits you with the death gaze, if you use the sphere it almost makes this fight too easy!

Trying to kill him but everytime some of my dudes becames a ghast!!! I tried dispel magic but it doesnt works!! What can I do??

Another interesting way around his Death Gaze is the Potion(s) of Mirrored Eyes: They aren't just for Basilisks.

One trick you might try is to send your best Tank down alone, at first. Warriors have the best Saving Throws, so they're the least likely to fall under his Death Gaze. Once your Tank has Aec's complete attention, bring the rest of your party down and start whacking Cultists. You'll need a Backup Tank or two, as your primary Tank will have taken a lot of hurt from Aec's melee attacks.

well to my knowledge if you have been hit with the death gaze dispel magic will get rid of it. It's kinda hard to see especially if you have a bunch of stuff on the character portrait from potions, spells, etc. But pause once and a while and look closely, if you see a little white ghost on one of your character's screens then you have been gazed. Simply cast dispel magic on that person, if that isnt working you might have a bug.

I had the idea with the tank work very well. i casted protectin from evil, aid and haste + potion of frost giant strength, heroism, oil of speed. then i just whent down with Ac -21 so th beast only damaged me on a critt.

Dispel magic *does* work - I've done it.
Just make sure that your priest & mage have a few of those memorised - you'll definitely need more than one!!