[Rataxes]

Hit'n'Run'n'Hide with a bow was what I always did vs Beholders when I played a thief or a ranger. Though it's damn tricky even with Boots of Speed and Oil of Speed since their reaction time is practically non-existant. Gauths cannot cast Anti-Magic Rays so if you're only facing those, or if you've already killed the Beholder, you can use a potion of Magic Shielding + Belt of Inertial Barrier + Potion of Absorbation (sp?) to become completely immune to their attacks.