So we did something similar to this!
Our party schemer concocted an ambush. We made a makeshift net / blanket thing out of bedsheets weighted on the corners. We soaked it in oil and the plan was two party members who could where invisible waiting to toss it over the beast and I was just inside the range of my Flaming Sphere - totally different than Fire ball - to ignite the mess. Best laid plans and all that - as soon as the beholder appeared and looked invisibility dropped due to a magic suppression thing it possesses. Which double sucked because the invisible sorceress we had poised to fling the oily tarp has a Daredevil type thing going - she was blinded in exchange for power, and her magical blind vision was now turned off. Her familiar / pet helped with attack and it somehow succeeded. My spell ignited it, and importantly, now that it couldn't see, our magic worked again. So a good round of us inflicting damage followed. My Flaming Sphere did damage initially and continued too for every round the beholder finished within 5' (unfortunately this also singed a few of my friends, with my damage rolls being annoyingly good against them). But eye-ball monster eye-beamed the cover off with a quickness and re-initiated magic suppression on us.
We chipped away at it, discovering those behind the creature could still cast and my sphere constantly moving back in to damage it. Mean time the creature flattens the sorceress with a petrification type spell, turning her to stone over the next few rounds. Our monk leapt into action, but missed a few strikes and the eye-guy made the save against stunning strike. Then she is flattened with a 40 hp blow, followed next round by another 31, putting her out. I'm stuck in the narrow passage in front of the thing, unable to cast anything beyond maintaining my sphere around and behind the thing. I sling some stones at it for very little damage, but the drake familiar flies into a rage when his master is fully turned to stone and gets in a good round. Then our rogue uses a homebrew magic quiver of chess-themed arrows. The queen arrow will automatically kill any creature with <100 hp on a hit. The bad buy made the save with against a 16 DC, but took the arrow damage. This allowed the monk to be slipped a healing spell and potion. She leaps up and has a great round with a flurry of blows that included a critical. More flaming sphere damage and the rogue scores another hit from his enchanted quiver that puts the beholder to sleep! The monk moves in and manages to finish the beast off after her first strike misses and the second only woke it up (monks, right)
Job done. The DM asks for a d100 roll (I got a sweet, solid bronze die for my birthday) for some reason. 10% chance he says. The selected roller hits a 100! We are all laughing at the odds of this (this is all taking place in a brewery at the end of the session. We'd had a few) Taking a breath, I ask "Is that good or bad?"
The DM, still laughing, says "oh, that's bad." Beholders reproduce by dreaming other beholders into existence. The slim chance we'd managed to achieve was ours doing this during the very short nap we'd forced on it.
I'm such a nerd. I love this game.