As some advice, the dragon can be knocked down and blinded, lower/nullify resistances work miracles as does the -5 willpower debuff and divine light. Curse arrow or con debuff arrows. And a 2 hander melee with wildfire, oath of descration, rage, flurry should be able to do tons of damage to him. (In some cases outright kills him).
It can even be Soulsapped! The Void Dragon does not have high CON - he simply has a high ratio of CON to HP. Soulsap dramatically lowers CON(== HP) and resistances. Combine it with a few more debuffs that reduce CON (Disease, special arrows, etc) and you can reduce the dragon's HP by more than 5k before doing any damage to it.
If anything, I thought the final fight was far too easy. The Void Dragon is not really resistant or immune to anything, unlike half of the enemies in the Phantom Forest. It's far too simple to incapacitate him for the entire fight.
They should have made Arhu (with his Hunter's Edge stats) the final boss.
