Get Jan and Viconia to cast Raise Undead if they can, the skeletons are immuned to the Mind Flayers Psionics and can tank for you.
Viconia can also cast Chaotic Command to protect your party members.
Be careful if you want to melee the Mind Flayers, or they will brain-drain you to death.
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If not a Sorcerer, then a Paladin.
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Baldur's Gate Item Mart