oh my god this battle is SO difficult, compared to the other fights, my party of K/M, jaheira, minsc, sarevok dualed to thief, imoen and aerie can't take this fight, and my party is quite high leveled.
The beast comes with a natural 80%+ resistance to weapon and my magics doesnt works even when i use lower resistancex3 x 30% and when he uses hardiness which probably is on a script then all my attacks heals him back, and add that together with his stuns/smites and 40damage attacks.

Cast "Breach" on him immediately each time he casts "Hardiness". Breach does remove hardiness.

Yup, but I figured another tactic; I used time stop+improved alacrity with vampiric touch; and he takes between 20 to 25 damage per touch, kinda nice. I noticed that the script summons guards even when time stopped, kinda strange i think...

Yes, it's how it works even in the vanilla game (i.e. even in the vanilla game, his minions keep spawning during timestop). I can revise it to pause spawning them while the time is stopped, but I don't intend to. I like it the way it is.

BTW, glad to hear that you found an alternate way to win the battle. There are always several tactics available for players who spare time to think.

Yup, it wasn't *that* tough, as soon as yaga fell all his army left the battle, less exp. but at this stage of the game exp. is a bit irrelevant.
BTW, I noticed that when I enter the oasis, all the npcs in the map comes after me, with something like 10 golems and 10 or so captains and battlemages; this battle is difficult; my party can't risk being overwhelmed.

The Oasis battle is one of those battles in which the difficulty slider in the game's configuration can make considerable difference. Decrease the difficulty to "Easy" and they won't come after you all.

Alternatively, you can (again) spare more time to find a good way to defeat them even on "core" level of difficulty (for more satisfaction).

That oasis fight was very enjoyable, as soon as the battle started my main protagonist casted time stop to start dealing melee damage (3 rounds of time stop promised the death of 2 captains and if my criticals were lucky the death of the general as well <the one wielding answerer>) then right after their deaths the golems came to pound on Jaheira whom drank a potion of barbarian essence, casted hardiness, and was wielding the helmet Gromnir, and wielding the staff of golem slaying got her up to absorb crushing damage; making her a perfect tank anti golems.

While there were 2 or 3 golems pounding on Aerie and Imoen on the back, which, wasn't a threat because they had about 20 scrolls of protection from magical weapons (which by the way I don't see any difference on this spell with improved mantle? a higher level spell...). The battle was extremely fun and satisfying heh all that on core level difficulty

The prot from magic wepon don't protect from normal weapons but the mantle protect it too.

Congratulation Tonpikls!