I ran this boss battle last night, and the whole group had a blast!
[Battlemap completely my own, made in Dungeon Alchemist, uploaded to and ran on Foundry VTT]
My party was searching for a mythical creature of great magical ability in order to ritually sacrifice it to save the god of magic -- inside the head of a massive giant they entered this mysterious "fey domain" that had a leprechaun ("Trickster champion of the fey") in it that they ended up needing to battle in order to sacrifice him. And yes, he had a bowl of lucky charms in the pot-o-gold that triggered the combat.
At two different pre-decided HP levels of the boss, the party was teleported to the gauntlet run on the outside of the map. I use Foundry and had walled them into the inner area so they couldn't even see the outer area -- it made for an awesome surprise.
I had this be a battle against a leprechaun, hence the massive throne and pot-o-gold in the center of the map...but I feel like this could be adjusted for most scenarios!
Gauntlet Mechanics:
[The gauntlet was inspired by one of the encounters in the Leviathan raid of Destiny 2: at various points, the whole fireteam is teleported to a circular gauntlet just outside of the arena where they would have to run together and survive. Failing meant going back into the arena to fight adds and hope someone survived the gauntlet to get leverage against the boss.]
I ran the gauntlet by running several "rounds" of movement: all at once, each character would roll a d6 (+1 for every 5ft of speed they have above 30) to represent how many spaces they could move that round.
I had them all move at once, which we could do with foundry. Everyone grabbed their token and hovered over where they wanted to end up based on how they rolled their movement, and then everyone moved on the count of 3. If anyone ended up on the same space, they would "collide" and fall prone, meaning they could only move half their rolled speed on the next round!
Alternatively, you could roll a second initiative and take turns, but going together kept things moving and added some fun chaos.
I gave them one round of headstart, and then I started rolling a d6 behind the screen to move a magical barrier behind them that would catch anyone moving too slow.
If caught by the barrier, they would be teleported back into the arena and held in chains while the boss is in a "trance" -- the boss would siphon 1hp from each person for each round until the gauntlet ended. It ended up being too much math to do quickly...I just took 5hp from each and gave it to the boss.
When/if people made it to the end of the gauntlet, they would teleport to the boss and get a free action! After everyone was back in the arena, combat would pick up where it left off.
1st Gauntlet:
Starting at the top left, the party ran eastward and then down the right side.
The top area just has pitfalls they needed to avoid, making an Athletics check if they wanted to leap over a gap more than one square wide.
On the right side, those three walls had spaces in them that could only fit one person at a time: the first has 4, the second has 3, and the last has 2, creating a scenario where people could get stuck trying to get through the same space at the same time. We didn't run into that scenario, but it did influence where the characters had to move in order to get through.
2nd Gauntlet:
Starting at the bottom right, the party ran westward and then up a bit on the left.
There are buttons to "turn off" the walls of fire, so someone would have to make a check to leap onto the first button. My idea was that for the second wall of fire two people would have to stay on the two buttons and a third person would have to stay on the button to the left of the flames so the original two could then pass -- but I nixed that on the fly. Whatever's the most fun, right?
Then I had some fun: I put 5 portals at the end of this run with the first letter of each character's name next to the portal specifically meant for them, but with the letter only legible if they passed a DC 18 Perception check.
Each portal led to a skill check catered towards that character: the one meant for Steve Irwin led to a crocodile the character would have to wrestle, and the one meant for Bill Nye led to a chalkboard with an equation on it. Those characters would pass the tailor-made encounter easily, but if someone else took that portal it wouldn't be so easy!
(I also added some real-world leverage: For instance, if the player could answer "What is the best way to get the crocodile to release you from its jaws?" with "putting your finger in its eyes" then they could roll the check with advantage.)
If they passed the encounter, they teleported back to the arena and got a free action on the boss. If they failed, I dropped them in a random spot a little ways away from the action.
All in all, it was a blast!! So much more fun than just standing there and hitting each other for two hours. It's a little chaotic behind the screen trying to run things fairly, but adjust to make things fun instead of tedious.
Please steal this idea! A lot of love and care went into it and I don't want it to just collect dust in my memories.