a pain and a SPOILER
I was on that part yesterday and had a similar problem - that I didn't think I had fought enough elementals - it was that the 4th elemental hadn't spawned and when I went back into the room, it did spawn and I led it off to freeze in the big chill cabinet.
the end boss (Chromatic demon) is very easy to kill and it is not mentioned in such places as the Prima Walkthrough!
keep the party back and have somebody with boots of speed release the demon. then - lead the demon to the poison gas chamber - the gas does a lot of damage to the chromatic demon - best against the air version, but affects him in ALL varieties. hastened, the character can run out of the chamber and across to the other exit to keep the demon in the room - if you don't care about the xp, you can simply shut him in - I think you need a door open to get reports of the damage he takes, but I'm not sure. it does not take very long at all.
killing the demon is the easy part - the slightly tricky part is for the character to deal the death blow, as if the demon is killed by the gas, you lose the 55k points. so - get the demon into 'near death' state and then hit it with the ring of the ram to do 30 points damage and kill it. the alternative is to lure the near dead-demon out of the chamber, but as it panics when it takes to much damage and has morale failure, you need to be able to perform the killing blow with enough power that there is no chance of it being finished off by gas damage. weapons do so little damage that it can regenerate faster than most parties damage it - but not when it is gassed.
"All the world's a stage and all the men and women merely players"