[--Twilight--]

Hello,
well, I can only advise on my BG experience, but, what I do is: pump-up the party with spells to increase saving throws, speed, and attacks, summon a big critter or two to get the fiend's attention, and then hit the beholder with every melee attack I possibly can. Their magic resistance is just too great to offer a line of attack. Furthermore, their powerful, neverending, varied magic attacks puts time on their side. Take them out post-haste!
I can't recall, off-hand, any battles where I held-off on the beholders while attacking other foes. Also, wounding them doesn't help you. Even if faced with multiple beholders, have your party gang-up on one, maybe two (with a high level party) beholders, and then move on to the next one. A long battle with beholders is one that has not gone well. They are awesome, in a twisted sort of way. As always, the single most important factor is knowing ahead of time of the coming battle.

- Twilight