What can I do to beat this beast? It's been 2 days and I am stuck. Any suggestions would be greatly appreciated

If you find all the documents around the level, they give you clues on how to defeat him, certain spells do damage to each specific form that the demon takes on, I didnt use much magic but just rushed in loaded with spell protections and pummled the crap out of him, and id switch to appropriate weapons that dealt the kind of magic damage that would harm his current form.

use power word stun on him and then you have a lot of time to just try everything on him. Thats what i did

I made him follow Minsc into the poison room, then closed the door on him. He takes a lot of damage, then keels over, no muss, no fuss!

Wow! Way to cheese and use the lack of noticable AI in the game. I only used cheese anywhere near that bad on my first solo (Swashbuckler/mage armed with Staff of Wizardry... Golems got 1 hit, then invisible... 1 hit, then invisible... lol, toss in fireballs and the like at will to spice things up! This is a newbs way of winning though).

I just sat by and watched those 2 and 3 damages pile up... oh, try to get FOTA+5 if possible, or any other form of it. Good chance of doing extra damage.

I managed to kill him before he had chance to do anything here is my strategy:

All you need is a thief with a high set traps ability. Set as many snares are possible infront of the cage he is trapped in (Rest if you run out of snares as they will still be there when you have rested).
Once you can set no more wait until he is in his ice form and then release him. As soon as he turns hostile the he will set off the traps and kill him self within seconds (They cause fire damage).

Hope this helps.

I created a couple of heavy helpers [effreti I think] just in front of the cage before I let the bastard out.

Everybody had bows or crossbows and a range of arrows or bolts. While he was dealing with the effreti I just kept hitting him with whatever type of missile he was vulnerable to in his current form. I had to add an extra summoned monster once when one of the effreti died.

I wait 'till he turned into his ice version and toasted him with my wand of wonders.

I just surronded him with my party and attacked, it seemed to work, killed him on the first attempt, although Anomen died