I am the DM for a group of 5 players who started at level 1 on the Lost Mine of Phandelver. Just last session they finished their encounter with the BBEG. The party consists of 5 level 4 characters: an anti-big-alchemy human druid (who is usually a wolf), a ex-pro-wrestler dragonborn monk, a tortured-past-criminal human rogue, a grizzled-yet-remarkably-charismatic half-elf bard, and a naïve-country-bumpkin dwarf barbarian.

For the last few sessions they have been smashing their way through the Wave Echo Cave; gasping and choking through the poisoned fungus room, taunting Mormesk the wraith with an orcish rock ballad to moonbeam him to death, a brief and nerve-wracking conversation with a spectator demanding to see visitor's badges, and finally, wiping the floor with the doppelganger and the bugbears digging for treasure down in the collapsed cavern.

Needless to say, the party was pretty battered at this point. Last session the monk got dropped to 0hp but was saved by the rogue short-swording to his rescue and forcefully pouring a healing potion down the dragonborn's throat. Everyone else was around half health, with a few spell slots left. The barbarian dwarf so far had the worst experience with Mormesk; she had lost about 1/3 of her maximum hp. Luckily her incredible rage was keeping her alive.

The Black Spider started the session by casting Darkness on the party and sent his 4 giant spiders skittering across the ceiling to web the whole party while they were staggering around in the dark. It took all 4 spiders, but the round ended with the entire party restrained. Nezzar grilled the party for their motives and backgrounds and forced them to toss aside their main weapons. The silver-tongued bard managed to convince the Black Spider that the party knew a better way to get to the Forge of Spells. With another spectacular deception, he says he needs his "wolfhound" (the wild-shaped Druid) to sniff out the entrance, and Nezzar, blinded by greed, agrees.

A spider snips out the two characters, and Nezzar calls for his two bugbears. One he has guard the webbed players along with 3 giant spiders, and the other bugbear accompanies him along with a giant spider, to escort the bard and druid (wolf) to the "better entrance". All is going well, and the mobile part of the party descends down into the collapsed cavern.

The minute they are over the ledge, the dragonborn monk (at only 7hp, mind you) begins slinging insults at the nearby bugbear to simultaneously enrage him into stepping into the webbing (triggering the spiders' web sense) while the rogue frantically cuts himself free. A confused giant spider drops down next to the bugbear and decides the web disturbance was a friendly, and the rogue manages to reach his weapon and re-arm. The dwarf barbarian then begins also cutting herself free using a handaxe.

Meanwhile, across the chasm the bard and druid are forced up the rope on the other side of the cliff first, Nezzar certain there are traps in the narrow tunnel. As the bugbear begins to ascend, the bard cuts the rope and he falls down. Nezzar immediately sends his spider up the cliff and casts Suggestion on the bard, magically telling him to throw down a rope or else his friends in the webbing will suffer. The bard makes his save but immediately flips on his deception and throws down one end of a rope for Nezzar and the bugbear to climb.

Back across the cavern, the dragonborn monk taunts the bugbear further, and the bugbear dives into the webbing and grapples the monk in rage. The rogue sees his chance and dashes forward, skewering the bugbear on his short sword. The monk then uses unarmed strike (with disadvantage) and manages to land a solid hit on the bugbear, knocking himself free. The spiders drop and begin attacking the webbed party, but at this point everyone has freed themselves from the webbing and immediately begin butchering the spiders.

Across the cavern, the bard then pulls the druid (wolf) back into the tunnel entrance and fires Shatter at the top edge of the cliff right as the bugbear and the Black Spider reach the top. Nezzar, his giant spider and the bugbear are all caught in the massive blast, fail their saves, fall 15 feet down the chasm and are hit by falling rocks shattered loose from the cliff. The bugbear is killed, but the giant spider and Nezzar both survive with 2 hitpoints. The bard pulls out his lute and both himself and the wolf take a flying leap off the shattered cliff. The druid wolf lands and crushes the giant spider in a spray of ichor and the bard rockstar smashes down, destroying his lute into Nezzar's face.

The dwarf barbarian finishes the fight by rage-smashing the last giant spider so hard it bursts into a cloud of fine mist spider-guts.

tl;dr: the party killed the BBEG by dropping rocks and a lute on him.

I can't wait for the next session. XD