Chaos has water powers that he can use to crush many opponents. Birdo has eggs, but they are no match for Chaos. Chaos can go to perfect form and pwn when he wants too. Birdo can’t win. Chaos wins.
Mephisto is a strong marvel being that can take on Birdo. While most would think Birdo could try throwing some eggs, I’m not convinced it would really do anything. Mephisto wins.
Buu has beams. He could easily beat Birdo. Birdo might try and suck up Buu, but it won’t work. Buu could easily beat Birdo with his aura. Birdo just can’t handle that kind of power. Buu wins.