Shao Kahn?
More like Sham Kahn.
They might say the Boos are a counter.
Maybe. There's also the question of if Mario has any in cyberspace or such stuff.
Most I can remember AtM is the ending to a MP8 Mini-Game where the Mini-Game's ending has a character left trapped in a computer.
There's also the Boo's Sheet item (Grants Invisible/Intangible status.), & the Spite Pouch which deals half the damage taken as counterattacks against Direct Attacks.
The former.... Well, Invisible/Intangible can't be hit, even by those that also have the status, AFAIK. Counterattacks ignore this.
Also, I'm realizing that funnily enough, Mario has tools Bowser doesn't here, because in TTYD, the Crystal Stars can hit "Invisible"/Intangible opponents, including intangible Boos, Dark Boos & those they grant it, too.
IDK if Star Spirit Powers can affect Invisible/Intangible foes, though.
In theory, Star Storm, Chill Out & or Up, Up & Away might work.
But even if they do, Bowser only "had" the Star Spirits in that he kidnapped them & sent his minions to oversee them.
Mario's use of Star Spirit Powers is literally through prayer & summoning. Unless Bowser gets the kidnapped Star Spirits & mind controls them or something AND they can hit Intangible....
Anyone wanna go down to post-Chapter 7 Crystal Palace & try Star Storming some enemies Gray Magikoopas Intangible'd up?
& while the Crystal Stars are already known to be able to bypass Intangibility, Bowser probably has even less chance of getting them, since the closest he came to having ANY was:
1. Fighting Mario. (& losing. Twice.)
2. Going to Rawk Hawk's Secret Underground Training Room to steal the Gold Crystal Star from Rawk Hawk's Champion's Belt, only for it to get knocked off its display on the wall, fall to the ground & break, because it's a fake.
3. Going to Poshley Sanctum and stealing a SECOND fake Crystal Star, because the real Crystal Star is in a secret second version of Poshley Sanctum with much worse lighting & lots of Dark Boos that you enter by going into a background painting of the building itself, inside the building.
Also, it was technically the third fake Crystal Star; The museum's curator, Pennington had to replace Poshley Sanctum's original fake because Beldam, Marilyn & Doopliss stole the first fake, & so Pennington had to replace it for public display.
But hey, if he had actually gotten the real Garnet Star, maybe Bowser could try Showstopper's Death Manipulation mechanics on Sage & company, lol.