Just posted an extensive reply about mindflayers in
this thread, so check there.
For liches, you can also use the upgraded Mace of Disruption +2, which, if you're in luck, will destroy them in one blow.
Item Spoiler:
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You can find the Mace in the Pool of Blood in Bodhi's Lair in the Temple District. To upgrade it, go to Cromwell with the Mace and the Illithium Ore you obtain in the Sir Sarles quest.