The Umber Hulks die to Cloudkill and Death Spell regularly. Summoning some Skeletons usually takes out the Illithids, or you can use Greater Malison or Feeblemind to lower their saves vs magic since they don't get saving throws vs those spells. Depending on you mage(s)' levels Abi Dalzim's Horrid Wilting works wonders saving throw or no! "Physical" magic attacks such as missles and temprorary weapons are also effective.
Certain NPC have abilities and/or equipment that can really even the odds too. Nalia has the Signet Ring that gives her a +2 to all saving throws, so she is a good candidate for an anti-illithid party. I think its Paladins that are IMMUNE to charm, add Anomen to your party.
You may also consider finding/making a ranger with Illithid as racial enemy. I recomend an Archer because this isn't the only Illithids in the game, and Archers own certain other Underdark/ground dwellers.
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