Help me figure out a BBE fight!

I have the final game for my in person game coming up in a week. Unfortunately, my timeline was shortened, so I never got to set up a fight with the BBE, but hey, here we are! And I'd love some thoughts on what I can/should do to make and keep it interesting. Barak, stop reading here.

The story is somewhat weak, and gimmicky, in large part due to the fact that it's all new players, so I didn't want to go over board. The game was mostly "adventure of the week" style, with an overarching story. The story so far: the players randomly met and ended up saving a village, finding this weird orb. Adventuring further, they found more of the orbs. They led to discovering a wizard who had captured a beholder, as he found that there are 10 of these orbs, and they open a path to a lost beholder treasure. The wizard brings the party into the search for the remaining orbs, and promises them a share of the treasure in return. They go on adventures, and find most of the orbs. As I start to have to cut the story short, we kinda skip ahead to the beholder escaping with all but a few of the orbs. The party goes to the location of the path/portal/door to try to stop the beholder, gets thoroughly beat, beholder steals the remaining orbs from them, and floats down a large tunnel in the ground.

So, now the question is how I wrap this all up. We normally play 2-2.5 hour sessions, the final will be 3. I'm thinking it'll mostly be a hopefully fun fight, and a little RP/exposition to end it. There are some B story lines that will be left open, but it happens. So what the party has going for them: they won the fight against the beholder's minions, but lost to it when it showed up at the end (they didn't fight it, it just GM won). Their allies in this big fight (a clan of deep gnomes) will clean the area up, and the wizard who's been helping them can do the magic heal/restore. I was thinking he uses some rare magic item to give them 10 hours in 10 seconds, so they can long rest, heal up, prep, but not be far behind the beholder itself. Then they head into the hole to fight the beholder.

So far for the beholder fight, all I have is the beholder being in the center of a large room, 10 pedestals around it, with an orb on each one. It uses an eye stalk to power each orb, and once all 10 are powered, the gateway/magic/whatever activates. In fighting the players, it'll have to take a stalk or two off an orb, to attack the players. I'm thinking the wizard ally will use his powers to keep the antimagic zone either nullified or minimizes, and reduce damage (the players are no where near strong enough to fight a beholder, specially a BBE). To add a flair to the fight, I'm thinking the beholder gets reduced damage, as he's connected to the magic in the orbs, but they players find that attacking an orb is the secret to beating him (somehow, not sure how yet, maybe just the wizard shouting something after a few ineffectual attacks?). I'm not sure if it should be attacking an orb he's powering, or trying to destroy the orbs he's not. But somehow that goes through, and after they kill him, they find that the process ruined the pathway, so no amazing powers for everyone, but an evil force destroyed.

I'd love feedback on the fight: what can I do to make it fun, interesting, not just a beat'em up. Should the beholder have minions or leave it a 1v4? Thoughts on the ending? Thoughts on gimmicks?

Leave it as a Boss Fight and let them all go after the Beholder. This scenario would benefit greatly from using Clock Mechanics...one piece filled in for each orb the Beholder powers, one piece removed each time the PCs interrupt the process. That should split the attention between wailing on the Beholder and de-powering an orb. If you don't think there's enough meat fur that grinder, add some PC assistants to let the tude turn back & forth for some thrilling tension.
If the Beholder is winning, let them win. Then reveal that the orbs are a test for some extra-planar creature that arrives as the obscure filled and devours the Beholder, adding its power to the creature's own. Unconcerned with the PCs, it recedes back across the planes to await the next Beholder to fall for the ruse.

Chiming in to agree with Witchdoctor, but I figure that the beholder is at full power at the start, but destroying an orb reduces its power by 1/10th, and also nullifies the ability of that eyestalk powering the orb. As a twist, each orb has some kind of relevant aura connected to the eyestalk powering it. So when the PCs approach a particular orb, they are subject to some kind of effect. This can spice things up because players have to decide which ones they want to neutralise first.