Heh.
I hadn't thought about Blindness, but I can think of a way to take down a Beholder using a single character, without magic.
This assumes that the beholder-victim lives in a rural area and has to go on long hunting trips. It also assumes foreknowledge and prep time for our hero.
Our hero is a Rogue, or even better a Rogue/Fighter or Arcane Trickster. He needs access to Sneak Attack, Hide, and possibly Wilderness Lore.
He observes the Beholder's habits, and on one of it's hunting trips digs a little shallow trench to hide in, lying face down or couched, right where the beholder will have to pass over it. Obviously, this uses the Hide skill, and possibly Wilderness Lore to seamlessly camoflage the blind. Then he waits. Eventually, the beholder will pass overhead.
He surprises the Beholder, and being underneath it, it can't use any of its attacks. It's also flat-footed, so he can use his SA.
The tricky part is avoiding the Beholder's spot bonus, but it's fairly easy to block Line-of-Effect to the interior of the blind, so magic is still usable within it. Scrolls of Silent Image, Cloaks of Hiding, etc will all still work. Given even more extensive prep-time, you can also condition the beholder to expect false alarms - it won't do it any good to spot the hiding place if it thinks its empty. Or perhaps you'll just have your captive blind kobold throw a rock at the right time to distract it.
Also, depending on your GM, the Beholder may or may not be able to fly upside down. Regardless, the safest place on the battlefield is directly "underneath" the Beholder, outside the arc of its eye rays. Our hero should attempt to stay there as much as possible.
You even vary this basic strategy (attack-from-beneath) with assorted mundane and magical traps. Or even just shove an entire adventuring party underneath.