Bowser, Mario, and Luigi will fight in this boss battle. I was thinking about making a sprite sheet about this boss, but I wanted to show you guys this entry of ideas first. If you like it and think this is a good idea, please comment or leave a star.
Bouldergeist will start off indestructible. Bowser's attacks should recieve a small amount of damage. In every few turns, Bouldergeist would eventually heal itself with more armor.
In Bouldergeist's Attack #1, rocks will rise (Mostly normal rocks, rarely gold rocks, and at least 1 black rock) and they will charge at Bowser. The rocks will charge directly at him, over him, or would miss. Nothing else happens when you destroy a normal rock. But destroying a gold rock will reward Bowser a coin or a mushroom, giving him HP. Punching a black rock would cause it to bounce back and lands on the right side of the battlefield, creating a black aura on the ground. After Bouldergeist is done with this attack, the aura(s) will turn into Bomb Boos.