I just made my second playtrough on hard difficulty and was dissapointed again with the final boss fight so searching the forum why is this I found this theme.
The dragon went down in the beginning of the 3rd turn with my two Lone wolf GC zombi rogues (no healing spells) :
1st turn- while ignoring the summons one rouge approaches the beast > Nullify Resistances > Oath Of Desecration > the other rouge also ignoring the summons blinds the dragon ( Eye Gouge ) for 100% Hit chance and also buffs himself with Oath Of Desecration.
2nd turn- 2x14 backstabs- the dragon is almost dead, meanwhile Astarte(?) is more then 2/3 health down
3rd turn- the 1st rogue finishes him.