Warmaster Horus
Explorer
That's a 150' cone with 120' eye rays. If the beholder is far enough away (90') and airborne, at least some of the characters should have a lot of difficulty escaping and doing something on their turn.
Beholders are also slow. A reposition might work as a legendary action, but a reaction might be a bit much (although don't let my thoughts on it stop you ).
As for running out of the cone, doing something and then running back into the cone, why cannot the beholder rotate a bit, fire its eye rays, and then rotate back?
This. A beholder will usually divert it's primary eye while using it's secondary ones. Players could dart in/out of cone on their turns to avoid Legendary eye rays but a Legendary Pivot could adjust to those moves at the use of Legendary action.