If you have the Staff of The Magi, that can really help your mage attack beholders, using its invis capability...
SPOILER -
5... 4... 3... 2... 1... I just had Edwin take out every beholder and gauth in the Beholder tunnel (in the Underdark) by himself without resting. He was invis with the S of Magi; as soon as he fired a fireball at a group of beholders he re-equipped the Staff to turn invis again - they never saw him.
[This isn't cheating, is it?] Robe of Vecna helps too - speeds your casting time... might need that as well.
...Also it didn't hurt that he got the mindflayers to help out - summoned an ogre, beholders and mindflayers both went after it; after the ogre died, they saw each other and both groups ended up killing each other after a long fight.
