IMHO, the toughest enemy is a Illithid.
Dragons get slautered after a few shots of finbger of death(reloading of course)
Liches can be killed with Cloak Of Mirroring
Demi-Liches can die by the hands of a berserker.
(in silent voice) If an illithid could be my friend! (smiles evilly)
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Coffee anyone?