After going through a second play through (again with pre generated characters aside from my main) I don't really think the final boss fight is overtuned at all. In a matter of fact, I would say it's tuned pretty close if not a bit under strengthed.
I feel that way because if you have a backstabber the entire fight is trivial; you can one shot him with your rogue on the first pass -> adrenaline, rage, elven active, vault, drink a potion of invis or stealth, then fatal blow (or whatever that move is called). Alternatively, you can have your 2H knight do the exact same combo and then do crippling strike/overpower and you'll come pretty close to killing him if not gibbing him outright. Also, Alexander has pretty bad magic defense so you can chew through that and CC him.
Beyond that all you need to do is take out the caster which you can dedicate 2 or 3 party members for and from there on out the worm will take care of one or distract someone and then your casters will have free reign for a turn or two -- unload your AoEs (chainlighting, spores, all the fire spells) preferrably from high ground - teleport your own casters there if you have to (which is what I did since it's 1AP). To deal with the Giest just drop hailstorm for 1AP and he's more or less CC'd.
All four times I've fought the encounter the worm has tied up 1+ npcs and even when it was just one I still managed to wipe the battlefield; my party composition was the following: Red 2H Knight, Murderous Elf Lady as a rogue, Loshe as a standard caster with most points going into Aero, and my main which was a caster with almost all points into fire. I had at least two characters with leadership 4 at all times. And everyone in my party had 110+ magic armor to counter blinding radiance. Beyond that, I would say if you're running for a long term fight you're probably better off using Bless to mitigate the shitstorm of necrofire.
