[Dar'tanian]

I would kill his pets first! And what you could do is back up your party untill you get to a point were you can not see the beholder(since he stays in pretty much one spot, or atleast for me he did)then cast an arial spell like fireball and cloudkill or higher level spells like that. If those dont work then once you kill hes pets go for him with melee and missle weapons and you can take him down easier.