Who are those other characters in the first room with you (blue circles)? When I did this amazing final sequence I brought Xan & Safana in addition to the canonical party, my protagonist, Immie, Minsc & Dynaheir, and Jaheira & Khalid. Xan and Saf fell in battle and I pretended they both got incapacitating injuries so I went back up to that room to collect the others, effectively making it feel like an 8 person assault.