In vanilla, if Thrommel got killed and you looted Fragarach, hid it away in a chest somewhere, then raised Thrommel, he would carry on as if nothing happened. And as long as you did not have Fragrach in your possession when your reward showed up, you'd still get Scather too. I seem to remember Iuz being easier in vanilla than he is in 7.6. My party in 7.6 is badass but he wades through them like they are nothing. Maybe it was that way in vanilla, but I don't remember it being so. I have Fragarach in the Ruined Tower chest waiting for the reward so I can get both. I think I may wait for that to happen before I take on Iuz. I'm looking forward to a chance to kill him with both swords having a go. I was thinking of using a weaker PC with O's Resilient Sphere case on her to run interference while I buff up the two sword wielders after Iuz appears. Can he break through that or does it stand up to the punishment. Does Lesser Globe of Invulnerability help at all?
Otiluke's Resilient Sphere works, but I've noticed my wizard still takes ability damage if Iuz sneak attacks. Lesser Globe of Invulnerability only works against spells.
Having both swords works very well, You could also summon a large beastie next to Iuz so that he hits that first. And then move your badass fighters in!
I haven't been able to kill Iuz in 7.6.0. Last try I managed to take off about 500hps I think, using one guy with Fragarach (max PA) and my other two fighters with +3 Holy Axio dwarven waraxe/longbow. I'm sure its possible, though, just a matter of landing more crits. I can't remember, is it possible to legitimately get both Frag and Scather?
How do you mean legitimately? If you kill the prince, take the sword, raise him and before leaving the temple dropping the sword then you can pick it up later and still be rewarded. I would call it a bug as i don't think he would reward you for taking his sword.