[Morgeruat]

Mind Flayers(illithids) as well as the nobles (ularithids) have a 90% resistance to magic, so cloudkill can work, but not well, it's more effective from a wand, as IIRC magic resistance isn't checked against wands in BG2. Death spell will take out the Umber Hulks, but not the Illithids, Skeleton Warriors are the most effective fodder against Illithids as has previously been stated, plus they have a high magic resistance, and lots of hit points, regular skellies don't have hp's, and other summons can be charmed. Prot from magic weapons makes you immune to their intelligence drain and drinking potions of genius can temporarily counter the effects of intelligence drain.

for Beholders the cloak of mirroring, if you don't have ToB installed won't stop their hold person abilities, or their non-damaging efects like charm, but it will still make short work of them when they use their cause serious wounds ray, the shield of balduran is a great weapon against them, using it and the hammer of thunderbolts (before i upgraded it) Anomen took out the unseeing eye alone without spells or potions, at about 9th or 10th level.