OK, here's a plan (note that its a plan, not the plan) that might work:
Have everyone delay actions until everyone can link arms hands (if you're close enough in the pitch darkness) and Alya can lead the bunch in the right direction - that direction is the middle of the chamber where the daylight spell is. At that point, you have protection from the beholder's rays. then the beholder has to shut its big eye to hit you with its lesser eyes. This should pose a problem for your opponent.
If the beholder drops the cone to fire its rays, then you will have light and magic again - use it, even if its only with readied actions because the thing shuts off the cone and then back on as soon as it can.
If instead the beholder decides to simply keep you ties up, work on getting non magical light sources lit in the safety zone and take the fight to the monster (with the grapple and sneak attack method - Darian looks like the is the prime candidate to grapple, as he's probably got some killer Fort and Will saves, failing that you ranger calrom fits the bill) - Fiddle and Vad should stay open to hit it with sneak attacks.
If you're somehow stuck in the middle with the cone on you and cannot get a light source, you've still got a chance before the flayer and grimlock swoop in to kick royal rump: Alya will have to play spotter. If your DM allows it (and its not too much of a stretch for her to play spotter for another character as a move equivelent action), Alya might be able to at least get the effectively blind PCs to fire missles/swing in the right area, which could allow for a 50% miss chance like fighting an invisible foe.
Have everyone delay actions until everyone can link arms hands (if you're close enough in the pitch darkness) and Alya can lead the bunch in the right direction - that direction is the middle of the chamber where the daylight spell is. At that point, you have protection from the beholder's rays. then the beholder has to shut its big eye to hit you with its lesser eyes. This should pose a problem for your opponent.
If the beholder drops the cone to fire its rays, then you will have light and magic again - use it, even if its only with readied actions because the thing shuts off the cone and then back on as soon as it can.
If instead the beholder decides to simply keep you ties up, work on getting non magical light sources lit in the safety zone and take the fight to the monster (with the grapple and sneak attack method - Darian looks like the is the prime candidate to grapple, as he's probably got some killer Fort and Will saves, failing that you ranger calrom fits the bill) - Fiddle and Vad should stay open to hit it with sneak attacks.
If you're somehow stuck in the middle with the cone on you and cannot get a light source, you've still got a chance before the flayer and grimlock swoop in to kick royal rump: Alya will have to play spotter. If your DM allows it (and its not too much of a stretch for her to play spotter for another character as a move equivelent action), Alya might be able to at least get the effectively blind PCs to fire missles/swing in the right area, which could allow for a 50% miss chance like fighting an invisible foe.