It's a King Krogmo Boss Rush Arena!
Costs crowns to enter. Probably a few thousand. Why? You gotta have stakes. Prizes are claimed when you die, based on how many bosses you defeated. (Alternatively, one may leave once they reach a terminal to claim prizes and quit the run.) Boss Brawl is always in Tier 3, and always on Elite difficulty. This means that lower tier bosses are a little beefed up (particularly the tutorial ones).
Bosses have no drops, so all of your loot comes from when you exit the arena (presumably a payout in Krogmo Coins and Crowns), and a terminal appears every four floors, offering limited healing. The players are allowed to start with a medium sized vitapod (8 perhaps?)
Boss fights are direct (no level to go through before the encounter), and the Shadow Lair bosses are fought "in order" (R. Snarbolax, Ice Queen, R. R. Twins, D. Vanaduke) as the final encounters. (There's also the possibility of an arena exclusive final boss, but I can't think of one.) All the bosses before the Shadow Lair encounters are fought in a random order, similar to The Arena in Kirby Super Star (Ultra).
(Here's the wiki's list of bosses if you care.)
Thoughts?
I'd like it, but instead of "limited healing" after battles, I would simply have 4 heart boxes. How much health you are able to get is determined by RNG, but you may find a better vitapod or health capsule in doing so. Prize wheel would have a chance to grant consumables as well, so keep that in there.
One idea to take from the slime casino of all places would be if you were given 5 "krogmo machine vouchers" after every boss that you could use to spin a prize wheel. The wheel would have health, consumables, and occasionally crowns or even a rare prize box or a bribe reward. Prizes get better and rarer the further into boss rush you go, but unlike the rest of the minigame they would be awarded upon spinning the wheel, not upon level completion.
Overall it would be a fun and interesting gamemode. A handful of rare or event bosses included would make the entire thing more interesting as well - imagine fighting a tier 3 margrel!