The same is happening to me, Serafen's companion quest and another late game fight. I cant kill them with aoe or anything. The last time it happened I just talked my way out of an encounter this one I have 100% repeatable unfinish-able combat. I PMd the save file as requested.
*edit After some troubleshooting it appears to be too much damage even if I let the enemies wail on me for a few rounds. If I one shot certain targets they go immune. To rectify the encounter I turned AI off on all my characters and slowly killed them with my weakest auto attacker.
