Is it possible to DD one of the fighters (or the sneaker) into an ideal strike position?
perhaps directly above the beholder (possible slam attack on the stalks?) sort of a Death From Above. The advantage to this is that if you hit, something gets crushed, if you miss, your out of eyeshot.
At the very least, instead of grappling the beholder itself, grapple the eyestalks (Using the DFA). Rip them out one at a time if necessary. they give you a great handhold to avoid being thrown and it also takes the creatures attention away from everyone else.
If you luck out and actually grab a stalk, there are many things you could attempt... the fact that the thing is floating essentially means that though it has mass it has no weight. if you are firmly standing on the ground and have a good grip you can beat it off the walls. sure it can try to counter by levitating against your swing but then, like the other attacks, it is distracted from everyone else.
the only drawback is that you become the primary target, but seeing as how you already are one, the tradeoff isn't that bad. and if you succeed.... think of the bragging rights!
One must never underestimate the value of style when attacking a monster. Sometimes an idea conjures up a picture in the DM's head so vivid that he'll actually allow it to happen.