Instructions
Collect a Star to become Invincible Mario. Stars are power-ups available by hitting certain boxes in the game.
Kill as many Boos as you can while invincible by touching them. This is the only way to kill a Boo.
Face a Boo to make it freeze if you don't have a Star. As long as you are looking at the Boo, it will stop and cover its face with its hands. As soon as you turn your back on the Boo, it will start coming toward you slowly. While this method doesn't kill the Boo, this is how you must get past the vast majority of the Boos in the game because you won't always have access to a Star power-up.