Never just rush a Beholder down. Always, always go in with a plan if you can.
Invert most of your normal tactics, don't scatter, or the monster will dissect the party, piece by piece. Keep the casters protected if at all possible, and the rogue would appreciate it too, since getting a sneak attack off on them is frickin' hard. And most importantly, it bears repeating...kill it first. It's true, no other enemy can sling that many SoD effects in a round, or that many crippling effects. Unless it's minions are breathing down your neck, ready to deliver mad beatdown, ignore them. The floating ball of doom is the threat! Always have some dispels handy, it charming or dominating your fighters is also hella bad.
Beholders have one huge weakness, and that's the fact that they have no defence whatsoever. They're the magical equivalent of a frenzied beserker, no defence, and can shoot themselves in the foot easily. Get right in their face, it's the safest place to be.
Oh, and never take one on in its lair if you can. A Beholder making pop-up (or down!) attacks from it's holes is a real nightmare.
I've only ever fought two beholderkin in my D&D career, both in Living Greyhawk. The first was a Gauth, and it was a real tough fight (damned Exhaustion ray!). The second was a true Beholder, but a first round Murderous Mist and a failed save reduced it to a flying medicine ball with teeth (For those unaware, Murderous Mist causes permanent blindness on a failed save. Who cares about 2D6 damage?!).
I know for a fact though, that both of those modules traumatised and killed a LOT of parties. They are awesomely dangerous foes.
Saeviomagy, that idea is...incredible. Why have I never thought of it?!