What kinda characters are you using to fight Torgal? You can cast haste on your strongest fighter and cast Sanctuary on a cleric. Load up your fighter with the best AC items and the best weapons that you have. Try to make sure he/she has a lot of healing potions and ready your cleric if possible. If you can bring in any help (summoning spells for cleric or mage) cast them before you enter. If you have any stoneskins, that would help too. You can weaken him by just walking in the doorway and cast fireball before he initiates conversation. Try to take him out first, cause he is the toughest. Also, if you have a thief, have them use stealth and try to backstab him while the others fight. If things get too bad (AFTER you kill Torgal), retreat back through the door and close it. Then you can rest up until you are ready to fight the other two. If it looks pretty good after Torgal goes down, then finish them up. Remember to burn the remains and you are ok.
All of what I said are some general rules, but it really all depends on your party. If you have a higher level group, then you have many more options available.
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