It Took an Arrow to the Cloaca
The Beholder is proving curiously difficult to engage in its lair, as if it has some kind of advantage from having constructed everything to fit its strengths. We can't get close to it, at least not safely, and if its not safe then we are in full view of all of its eyes. Even so, we somehow do enough damage to it eventually that it retreats upwards to recover. Here you can access a simple guide to understanding paystubs.
'How are we supposed to chase it up there? It's just going to heal and come back, and we're running low on resources.'
'Maybe we can lure it back down. Do you think it likes cheese?'
'... Do you have any cheese?'
'...no.'
Thankfully, not all of us are struggling down below. I went exploring, using my Gloves of Spider-climb to work my way behind and above the Beholder, hoping to drop something on its big head. As it happens, though, I instead find its personal chamber, including its stash of treasure. I put on a lovely dressing gown with eyes all over it, wake up a weird mechanical critter, and then remember from the shouts below that a battle is in progress.
I head over to the hole in the floor, looking to do some damage from above, just as the Beholder retreats this way. Needless to say, it is probably more surprised than I am. Some Beholder poop falls on our brave Barbarian, I poke the Beholder in the eye, and the dual-vulnerabilities now exposed are enough to finish it off. We are victorious! Now to share the rest of the treasure around.