I'm curious about the people who claim this boss is "simple" and "easy"

I'm prettty sure they're the ones who quicksave and load every fight, every roll.

Of course it's easy to exploit.
Exploit books.
Exploit godmode (quicksave)

And that's exactly how I beat it (but never read or bought a stat book, fyi)

There was no way in another 90 hours of gameplay I was going to beat this boss with my lowly party of 3 who had barely made it to level 20. Having been forced to complete areas of the game that were pointless just to get there was in itse;f a tedious and anti-RPG element of the game. Sandbox? sandboxes don't force me to do everything in game. Optional quests should be exactly that, and there should be equal experiences to killing stuff and quests, rather than 95% of all xp coming from mobs.

But onto the strats and stats.

My Party:
Lame tank (Cain, who I bought at level 1) with stats that gave him 200 armor
Lame rogue (equipped with snakebite(from Mangoth) that she never used, and a level 11 bow) who had about 10 intel, 3 points in witchcraft, full and some points in
LAme archer (had a nice legendary lvl 16 bow) with disease and some other things, plus a poison harness or something

No one in my team had tenebrium ability (never found how to get this) but snakebite was a non-rot tenebrium weapon

NB: Up until this fight I had always played fights 'fair' i.e. like DA:O, no saving during a fight
This included things like balberith and that other demon Raz k'ad or whatever (hard bitch if I say so myself)

other equpment: a billion arrows and scrolls I'd picked up in my journeys
Strat: Chase Trife with my tank (slow bastard who i got to 9 AP)
Kill adds with rogue+archer.
Whittle down trife
when near the end, use potions to heal team (had about 50 health potions)
kill trife
Dragon didn't spawn until last add killed
Save.
Charm: Spider, and other melee thing
Save
Knockdown dragon (man at arms knckdown 3 tunrs)
Save
Create: summon (skeleton, elemental etc)
Lay into dragon with spells, consitution arrows (-1k hp), resistance nullfieds
Once dragon was down to 4khp I played 'fair'
every -con arrow fired when i could, - armor, - reisstance etc
use soulsap enfeeble, scrolls with high dmg (poison spores), death punch.
Had 5 bloodstones before fight - used 3 with no cheat save/loads in between
Archer + rogue stunned for about 10 tunrs each.
Win game, eventually.

Total time: approx 2 hrs.

Enjoy