This dude took about 15 tries before I realised the main thing stopping me from killing him was the accuracy debuff
Eder - full deflection tank, away from main party, lots of endurance potions.
need: massive buffs to accuracy and immunity to paralysis and stun (thanks Durance you crazy old coot)
Get Eder to attack an ice blight and turn off his AI. keep him alive as long as possible (armor with 2nd chance can help).
DO NOT ATTACK THE DRAGON
kill off as many stray ice blights and oozes as possible without getting too close to the dragon (their damage is not that important)
get Eder to attack the dragon. a butt load of wraiths and spectres will appear. destroy them with your mad buffs and what not. prayer against paralysis is a huge help here.
hopefully Eder is still alive. take some time to rebuff your accuracy and might (prayer against fear doesnt seem to help too much as the dubuff is reapplied instantly)
smash dragon as fast as humanly possible and pray to Berath. keep him paralysed, petrified, stunned as much as possible (gaze of adragan, chaos orb, scrolls of paralysis)
THEN I realised scrolls of paralysis are craft-able, spam those as much as possible when you're going for the dragon kill, easy mode. then I was sad cos I realised I wasted 8 hours of my life NOT crafting these scrolls.