This was a tough encounter but doable with tactics. If anything the game could do with more tough encounters like this one.
It sure is disappointing to have to fight the drow after the beholder is killed. The beholder keeps the petrified drow around so that it can charm them into fighting for it, but they're hostile to it- it's odd that it doesn't try to charm or petrify your party though.
It would be a great encounter, dripping with juicy lore, if you could save the drow and uncover their story. Even if it most likely turned hostile afterwards, it would be nice to have the dialogue.