[SixOfSpades]

The Way of the Warrior:
Use somebody with Stealth to get a single Beholderkin just within their visual range. Fire 1 shot, run like hell, and immediately go back into Stealth. The Beholder will have time to fire 1 shot at you, though, so it helps to have the Cloak of Reflection (NOT Mirroring), as their Lightning Bolts will prove ineffective. You'll still get whacked by their Cause Wounds, though, but their Hold and Slow you can pretty much ignore. If you happen to have a Paladin and a certain sword, you can have them do the shooting, equipping the sword just as they turn tail & run. Yes, it's time- and resource-consuming, but it gets the job done, because Beholders don't have Healing potions.

The Way of the Wizard: (Easy)
Polymorph Self. Check out the benefits of the various forms.

The Way of the Wizard: (Sexy)
Protect a Wizard with every buffing spell under the sun, including a healthy dose of Spell Immunity. Then, and this is important, Spell Shield. Ignore the description on Spell Shield--its ONLY use is to grant immunity to those #$*(@ing Anti-Magic Rays. For the entire duration of your Spell Shield, the Beholders will be unable to remove your buffing spells.

I've found that Summons are useless against Beholders--their infinite Death Spells means that all Summons except Gated ones are toast. Gauths, however, do not have Death Spell, so Skeleton Warriors would still do the trick there.