Just hit Jareth on my first playthrough and was severely disappointment to lose my damage powerhouse character to an unbeatable defection! With my party at 75% the encounter was severely overtuned - I'm not a guide-reading min-maxer. I could kill Jareth and might have had a slim chance at an overall win, but once I saw that Jareth's death didn't bring Bearface McArrow back to my side I just triggered combat without talking to him and beat the encounter fairly easily. Tips: the summons can be killed with Destroy Summon, and using scrolls to dump your own summons on the battlefield allows you to burst him down fairly quickly.
My residual complaint is that this approach does not allow me to finish Balty's questline, and indeed she still gushes about her former mentor as we stood on top of his body. It feels either sloppy (broken quest) or insanely unfair (defection). My immersion is temporarily frustrated, as my party member mowed down her former mentor/father without noticing he died, and then continued whistling on her way.
Overall still a great game, but this part could use tweaks to either make defection part of dialog choices (instead of personality traits) and hint at the consequences. Also, add some recognition of killing Jareth (e.g. having Bairdotr mention that she was mourning him but understood that he was evil).
