If it were a real Beholder, it would wipe out the team in a second

This is by far the toughest fight in the game. The Spectator is able to exclude two party members from the fight and frees the Drow every turn.
Interestingly, from patch 4, the fight is broken before Drows attack randomly by team or Spectator. Currently, they are attacking only the team.
It is definitely more difficult and you need to burst the boss to win the fight.
I hope they'll fix it in the next patch.