My players enjoyed it, but it ended up far less dynamic than I'd hoped.

From what I've learned:

  • - Keep that sucker mobile. Nothing blahs it out like the ball of beams just hanging out in one spot. If the players want to nail it down, make them work for it. Beware of Giant Ape polymorph.
  • - Minions (or adjacent small enemies) keeps things active. You can't just ignore the ground game. If your minions aren't allied with Big B (or even if they are), having them as part of the collateral damage is on brand and a little fun.
  • - The antimagic ray makes a good penalty box for casters. Let the martials mull about and get eyebeamed while the beholder forces the party to move around. If you have an easy way to represent the AM space, (dynamic lighting, pipe cleaner cone, whatever), it can help you and your players keep track of where to move.
  • - Make sure there are things to climb up, hide behind, manipulate, and destroy. A nice disintegration ray turns your full cover into an empty space.
  • - DO NOT FORGET LAIR ACTIONS AND LEGENDARY ACTIONS! With or without minions, this helps rebalance the action economy some.
  • - Roleplaying. Be the Giant Floating Ham. Go nuts with your mania. Chew the scenery and milk the giant space cow. Let the players goad, cajole, flatter, deceive, negotiate, or threaten during combat. But keep in mind it doesn't have to stop acting and attacking just because it agrees to listen to the players. Startling a beholder is a great way to get a random beam shot at you.