In terms of tactics, a sensible one of the Beholder's side is using verticality. Floating up a or down a long tunnel forcing the PCs to follow, and the Beholder can easily keep them in his anti-magic cone and out of cover. Particularly if the Beholder needs a means of retreat, as without magical flight following may be impossible for many parties.

On the PCs side, particulates are their friend. Smoke, dust, fog and mist are all great ways to even the playing field.

Beholders make great villains because they are insane enough that you can justify pretty much any course of action they will take. You can play them as extremely clever, their paranoia making them have a prepared answer for everything the PCs can come up with. Or make the paranoia their downfall - they are so caught up in it they jump at shadows and assume the obvious solution is a cunning trap, leading them to make huge blunders.