Just remember, it can fire all of it's eyes in the same round, just not all at the same target. You drop it suddenly in the middle of the party.....
But if that doesn't work for you, you can always have the beholder using mirrors....
Also, the middle of the beholder can be the border of the arc zones. Meaning that the 3 pcs down the hall and ever so slightly to the left get 3, and ever so slightly to the right get the other 3....
Or, just use multiple beholders for that extra eyebeam action...