Dont tell him to take the bards way out and come back later. Talk to mencar and tell him no trouble your going. Now when you get close to him he wont say anything and you can prepare right next to him if you choose. Cast all your buff spells and most importantly stoneskin for aerie and ironskins for jaheira. Hide your thief in shadows then have jaheira start casting entangle and backstab pookie. You can take the mage out in one hit but then pookie will get his protection spells up immediately so take him out and this injures the mage from his familiar dying. Whichever target you choose once your visible run back to the stair case and have aerie use wand of cloudkill and minsc block up the doorway. By the time your thief made it back jaheira and aerie will be done casting and can plug up the rest of the doorway for a bit because of their skin spells. Use direct damage spells and hold person and missile weapons to finish them off and make sure to kill brenan risling before he escapes.
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Welcome to hell.