Has anyone completed this quest and know how to beat the thralls?

They seem unusually overpowered in some way, even for my level. Perhaps I am missing a key element to weaken them?

I have tried the earplug thingies but it says "invalid target"? Am I to somehow target the Siren when I can't?

Any help would be appreciated. Have resorted to hiring a friend to standby at the campfire to revive me all the time.