A few suggestions
As a huge kirby fan, I think that this could be a little more authentic. so i decided to list off some things, if they're possible.
1. Random boss order. With the exception of the final bosses, the boss order should be completely random. (This was the case with every true arena except Planet Robobot's)
2. Rest area. Between bosses, you would be taken to a hub, with 2 random shields to choose from. the hub would also display which boss you'd fight next, and how many more you have to go.
3. Shield armory. Before starting the boss rush, the player can choose a shield to bring with them.
4. Challenge mode. Go through the entire gauntlet with only 1 life, and no way to get extras. Health would refill between boss fights.