The fact that their insect plagues have an infinite duration plays a no-small part in that just so you know. I won those fights by avoiding letting anyone but eder get hit by those buggy bug spells (Get it, cause they're buggy and it's bugs, hoho)
How did you kill Raedric, his three Guardians, his paladin, his priest, his bodyguard, two archers and the two archmages in three rounds? I'm really curious about how you managed that.