[IronDragon]

The simplest solution I have found is to shuffle most of your party out of the way and then equip your mage, Nalia in you case with a wand of couldkill and a pile of invisibility potions. The cloak of non-detection is nice but not totally necessary. Turn your NPC AI off other wise Malia will try to tackle an elder orb with a short bow. Turn Nalia invisible and have her go beholder hunting. When she finds them (they do tent to hang out in groups) have her back off till she is out of their visual range. Make sure there is corner or side passage nearby. Fire off the cloudkill and book around that corner drinking the invisibility potion as you go. Yes the beholders will come hunting for Nalia but they can’t find what they can’t see and failing to find her they will obligingly go back into the poisonous cloud. Repeat the process about two more times. Usuaally after three times only the elder orb is still alive and since he, she it or them has high magic resistance the cloudkills did little to harm it. But since it is now alone your party should be able to handle it.