Beholders also don't get along very well with Fire Elementals, which I often use to great effect against them as my main battle tanks whilst my other fighters used missile weapons. The further you are away from them logically the less damage you'll take from their nasty rays, so smart usage of summoning spells and knowing when to rush in should see you safely through.
I never did get myself the Shield of Balduran to check out just how cheesy it is...that's something for me to do next time.
