Beholders are indeed a challenging foe. But there is a way to kill them that is SO easy, its almost unfair. Get your rep up, the Ring of Human Influence, and download the bonus merchants (if you don't have them).
Then go to the Adventurer's Mart, and from Deidre purchase the Shield of Balduran. Equip it on your lead character, and simply walk through areas with Beholders. They will kill themselves nearly instantly!
The only spot in the Unseeing Eye area to worry about is the Blind Priests in the centre, but they're a pushover with a Cloudkill or two. The Unseeing Eye himself is a mage, and has no eye attacks, so you can use standard anti-mage tactics (you can kill him without the rod, but it takes longer), or the rod and a thrown rock...
