King Boo (The Ruler of all the ghosts and Luigi's dire enemy)
World 9: Bowser
In the first part of the fight, ol' spikeshell sports a new Koopa Klown Car, complete with automated mechanical arms (Sonic 3 final boss, anyone?). While he stays above you, virtually safe from harm, he tosses in Bob-ombs and summons Micro Mecha-Bowsers, while staying above the battlefield, laughing at you while you either get blown to bits or killed by his windup toys. To end this phase, hit him with either the Bob-ombs or the Mechas three times to drop him off his little toy and into the lava (why does he put his castle in a volcano anyway?!)
He comes back to guard Peach in his bony form, tossing bonemerangs (like the Boomerang Bros.) and spitting blue fireballs. To drop him, simply stomp the switch behind him to drop him into the purple goo.
The form first seen in Brawl returns in this final phase: You must climb out of the crumbling volcano, while Giga Bowser chases after you, spitting fireballs and occasionaly swiping his tail; the strategy is simple: just RUN; don't even think of fighting him, just run to the big switch at the back of the volcano to bring him down.