I have successfully beaten the tatooed man at the Friendly Arm and his little army of wind horrors and undeads... one by one, thanks to this nearly-invincible Flamming Fist Elite's unit with great thac0 and unlimited cure critical wounds spells... although he failed once his morale check, and I was in big trouble

I have also beaten the high sorceress on the top of Friendly Arm Inn without too much pain... thanks to my wands.

I am now in Beregost, teleported into that large room. I awake in a corner, far from Teven and the two high sorceress. A Big Thanks for that, it allows me to save right after the cutscene and before the battle (it's bloody annoying when you have to watch a cut-scene or choosing dialogues options before a battle at each reload).
But I cannot kill any of them. I can even hardly hurt them before I bite the dust.
Teven have lots of hit points and a extremly good Thac0 : even with ctrl-y the two sorceresses I can't kill him without loosing most of my party. And with the sorceress and their mass dominate spell, I am really helpless. I do not forget their globe of invulnerability (my dispel magic failed to dispel those), which made them immune to most of my magic. My wands did nothing either.
I have tried to drink two potions of magic resistance to get 100% immunity to magic, but with the sorceress's ability to teleport away and attack my group, this is not a winner strategy. I am thinking to either a) drinking my all 10 potions of magic resistance and using one protection from magic scroll to get all my party immune to magic, or


Any hint for this battle would be very welcomed.

PS: I am playing the Bg1Tutu version.