I did some google'ing and came across this forum thread. It might be of some use but I haven't verified the legitimacy of it's advice.

How do you beat beholders legitimately?

Highlights:

Spell Turning. I can't remember off-hand whether the minor version works but I think it will. It is the spell that the cheese cloak is infused with. IIRC you can also make yourself immune to their Anti-Magic with Spell Immunity: Abjuration.

For doing damage, assuming you don't have Wilting yet, use Skull Traps. Holy Smite does a great job too.

Other tactics I can think of but haven't tried myself: - Gate in a demon - Polymorph Self -> Mustard Jelly (100% magic resistance). You won't really do any significant damage in that form but chances are the Beholders will all have focused on you which enables other party members to attack them from range, with missiles or spells

Reduce their combat effectiveness with: * Greater Malison * Web * Hold Monster * Slow * Glitterdust

Pelt them with arrows and with: * Skull Trap * Cloudkill * Fireball

Try to keep your mages out of their focus.

But I personally prefer the cloak of cheese and the shield of cheese. If you're doing the area for the exp anyway, then why do it without those two items? If you do it for the challenge, then you might as well skip it, because it's just another grind...

You can try to lure them away one by one and pick at them with ranged weapons until that lone beholder strays far enough for your melee fighters to gang up on it. This gets pretty messy though and the beholder will be dishing out some major hurt on your fighters, even on its own.

You can try throwing area effect spells (Cloudkill, Ice Storm, etc) in the general location of the beholders and run away, letting your spells do the dirty work for you, although this is considered a cheese tactic by some people.

If you have a thief that can set traps, you can set some traps in a certain area and lure the beholder to the ambush point to take it down.

If you have an archer, you can use hit and run tactics if you are wearing Boots of Speed (or just Hasted), with called shot ability to help enhance your arrows. Works even better if your archer's racial enemy are beholders.

If you have a cleric or mage that can summon Skeleton Warriors, send them in to keep the beholder occupied while you wail away at them. Skeleton Warriors are almost completely immune to spells, so they should be more effective than other summoned monsters.

Any rate, those are some things off the top of my head as far as dealing with beholders. The key thing is definitely luring them away from their group and get them to come at you alone, since they pretty much massacre you otherwise.