Bowser

Mario Boss(3): if killed, incapacitated for 1 round, then restored to full health. If killed 3 times, the boss is killed.

Mach Punch: Melee Weapon Attack: +12 to hit, reach 15 ft., 1 target. Hit: 56 (4d12 + 28) bludgeoning damage. 

Punching Hats(4 hats): Ranged Weapon Attack: each has +4 to hit, range: 80 ft., up to 4 targets. Hit: each hat does 28 (2d12 + 14) bludgeoning damage. If any miss, and someone succeeds on a DC 10 Strength check, they can wear the hat and run 40 ft to attack Bowser, dealing 28 bludgeoning damage.

 RIng of Fire: AOE: DC 10 Dex save, range 0-100 ft in one round. Hit: 18 (3d10) fire damage.

Bowser has 5 legendary actions, which he can use at the end of a player's turn. They reset to 5 at the beginning of this monster's turn.

Jump(1 Action): Bowser launches himself and anyone he's holding 30 ft into the air, landing up to 60 ft away and using Ring of Fire.

Block(1 Action): Bowser places his arms in front his body defensively, increasing his AC by 5 and decreasing his Dexterity modifier by 4