I will say Marx seems like a strange choice for a boss, as he’s more of a side character in the Kirby series now.
But he transforms into a monster for both Classic Mode and World of Light. This incarnation is heavily based on his boss form from Kirby Super Star.
He’s got a lot of quick and fancy moves.
And some pretty darn cheap attacks like giant laser beams.
But he can be defeated kind of with ease. I will say, he’s one of my least favorite bosses in the game.