In the absence of a better plan, let's have Weaver and Ulthok teleport the rest of the party either behind or to the side of the Beholder.
It would be a good idea if as many of us can be made invisible as is possible (perhaps improved invisibility on Ulthok and Weaver and the two remaining invisbility spells on whoever you think is deserving of it).
I'm not sure if we'll be able to anything other than range attacks on the beholder, and we run a constant risk of hitting its human shield even then. Also, we will be exposed to its stalks. But, it still seems like the best plan.
No, enough time is not the issue. Even waiting another week we are still going to have folks doing what they have posted; there is no more planning to do. I would like to get the attack started so we can at least move on.