Raise Undead are great as Tanks against Mind Flayers but Umber Hulks rip through them like tissue paper.
Basically, cast Chaotic Command on a party mage and do the following rapidly.
- Open door.
- Cast a Cloudkill/Death Fog spell.
- Close door.
- Wait.
The spell will instant kill any Umber Hulk in it's area of effect.
And may injure the Mind Flayers too. The reason why you need the Chaotic Command is to protect your Mage from the reaction psionics from the Mind Flayers which can pass through walls and doors.
Once the Umber Hulks are taken care of, use the Raise Undead spell to summon an undead army and let them tank for you while you hit the Mind Flayers from a distance.
Greater Malison and Lower Resist followed by Chromatic Orb is a great combo if your mage is Lvl 12 or above.
Tip:
If you want to find the locations of the Umber Hulks, Wizard Eye can be used, simply summon one, open door and fly the Wizard Eye in, the Umber Hulks and Mind Flayers will not attack it, just be careful not to let them see the person opening the door.
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If not a Sorcerer, then a Paladin.
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Baldur's Gate Item Mart