The
barrage ability really doesn't seem to make much sense. Rather than that, I'd like to see some form of special area attack which the Beholder can use in place of its individual rays, by firing all eyestalks at once. Something powerful enough to be worth giving up ten ray attacks, but that it wouldn't just use every round (or maybe give it a recovery time, like dragonbreath). How about:
Arcane Barrage (Su): As a full-round action, a Beholder may unleash a simultaneous barrage of arcane energy from all of its eyestalks at once. This ability blasts a 60-foot cone with disruptive energies. Creatures caught in the area suffer 10d6 force damage and are dazed for 1d3 rounds. A successful Reflex save halves the damage and negates the dazing effect. After unleashing this barrage, the beholder requires 1d4 rounds to recover its arcane energies before using the ability again, though it can continue to use its eye-rays as normal.
The damage could probably be bumped up to 10d8, but I'm being conservative.
I also agree with the suggestion to give the beholder a targeted
dispel magic as an eye-ray.