Here is tactics I've used (not the best, but working):
Cast Chaotic Commands on one character, equip him with a bow, nice arrows (of piercing, f.e., or +2) and Boots of Speed. Leave the party somewhere aside and go to MindFlayers with this lonely character. Then do a lot of shooting&running around. When Mind Flayers are dead, the hero can kill Umber Hulk (there are usually a couple of them with MindFlayers) with melee weapon.
Whatever tactics you choose, I hope you will destroy all of those beasties soon and successfully!
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Tanda,
with best regards from Guild of Assassins