Well... I just done that area with my Necromancer.
I loaded a Spell Sequencer with 3 Flame Arrows and instant killed the Beholder. With the Beholder out of the way, Korgan and my Swashbuckler attacked the Elder Orb, Korgan disrupted its' spell-casting with Frostreaver( elemental damage), while William stunned it with Celestial Fury.
Azrael hogs the battletext window.
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If not a Sorcerer casting total oblivion, then a Paladin meting out justice. Otherwise, I think I will just be a bard and torture my enemies with, "My Heart Will Go On".
Platinium Sorcerer of ALSB,
Aerie
Loving
Sacred
Band! She's MeowRrrr, and more!
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Baldur's Gate Item Mart