De'Arnise Keep Options

[Olbas]

I am stuck in the Keep and was hoping someone could help me. I went down the keep stairs to confront Torgal but was unable to beat him(didn't try much though because still stuck on Greater Yuan-ti) and now I can't go back up the stairs. I can't find anything in the Documents about this so was wondering if it was a bug or possibly it happened in the vanilla game but I don't recall it happening.
Also I can't beat the Greater Yuan-ti and if I kill Torgal first then will he disappear and I won't be able to forge the flail?

Please can you advise me how to beat the Yuan-ti,

Party:

Kensai-11 Dual wielding Long Swords

Berserker-11 Bastard Sword and Shield

Cleric/Ranger-9 Dual wielding Flail and Hammer

Cleric-10 Mace and Sheild (Is a swash dual but too much experience away from getting those skills back)

Jan-10 mage/11 Thief

Edwin-11

Normally I have my fighters kill the spider and Jan and Edwin cast Breach, Remove Magic and Spell Thrust on the Yuan-ti and then send my fighters in to attack him. I continue Breaching to remove any stoneskins he casts however I can do hardly any damage too him before I am killed, the one time I got him to badly injured he cast protection from magical weapons and I lost. I'm not sure how I can beat him, I have tried slightly different tactics but this is the best I have so far.