4 level 10's killed a CR 15

To make a long story short our party, a couple players were missing so we had 4 people. We are traveling to kill an arch hag. Basically a archfey who's a hag. We got stuck trying to stealth around the "Indominus Rex" that our dm homebrewed. My character, goliath shadow monk, with a few levels in druid, used Thunderwave to try and lure the thing into a trap.
Kind of worked but said trap was a gargantuan plant monster that could not hit the monstrous Ac of the beast. So in about a turn it tore through 3/4 of the plants hp.
Our changeling sorcerer/rouge, Courage, cast slow on it, it failed but used a legendary resistance. The issue with this is that we were in an area of wild magic. Any leveled spell had a chance to proc a roll on the dm's wild magic table. And what pray tell did our hiding rouge get on the table? Fairy fire, making her a glowing beacon.
The beast rocked up and downed her in one turn, then swallowed her. At this point she was dead.
Elizah used her racial ability to become large, drew the beasts attention. It was just far enough that it ended it's turn 10 feet from her. On her next turn she used step of the wind with a focus to dash and disengage. Trying to lead the beast back toward the damaged plant monster.
The beast instead went for the paladin, who through happenstance had become the Jesus of the campaign.
Paladin of redemption, Elyas, was nearly killed by a claw, which grappled him, and a bite attack being left with only 3 hp.
Pulling out the dagger of hurts so good, he decided asked our dm if he could willingly crit against himself.
Our dm a little nervous allowed it. The thing about this dagger is that it can turn what should be damage inflicted into temp hp. So he crit stabbed himself in the neck and smited himself.
Gaining 43 temp hp.
Using his other attack as a shove he managed to escape the grapple. getting hit down to 12 temp as he took an attack of opportunity to escape.
Elyas, pulled a scroll of banishment and managed to banish it away.
Before that our other pure druid, Ron-mar, had cast sleet storm on top of the beast.
My character the monk, Elizah, cast darkness on it, and marched into melee to wear down its hp. She didn't want to lose Courage. Managing to get a few hits and chip off some hp.
After the beast was gone, we had to decide if we were going to try one more thing to get courage back or flee.
We choose to give it one more try and if it failed we would flee.
Ronmar asked Elizah for the bombs in her bag of holding, Gunpowder kegs. In basics we gathered 5 kegs of gunpowder, 8 vials of oil, 2 jars of psi goo, and an alchemist fire to light it.
5 rounds after it left Elyas dropped concentration and the beast popped back in.
Ron-mar now a giant ape picked up a barrel and attempted to shove it in the beast mouth. It took everything we could muster but we managed it.
To which Elizah lit the pile by throwing the vial of alchemist fire.
Our dm calculated the damage and it came to about 256, 3 more than was needed.
Our dm rolled to determine if there would even be a body to recover. Fortunately there was, but she was missing her left arm and right leg.
Elyas rushed in and cast revivify.
Dm asks Courage what they want for their character. Did they make it, is she too far gone, or roll for it.
He chose to roll for it having 50/50 odds for each. Rolling a 14 Elyas made it in time.
We searched for her missing limbs but only managed to find the leg, which was reattached with a powerful 40 ish hp cure wounds.
And that is how our party of 4 level 10's killed a cr 15 monster.
Our dm had been planning for us not to fight the beast, and for it it be a recurring encounter while we were stuck on the island with it. Needless to say, that plan went our the window.
Chaos killed our rouge and chaos got her back.
Oh and one more thing, the name of our party.
Murphy's Outlaws.