Perhaps the most disturbing of the signs, you have to rely on your wits to get Mewtwo.This large cavern is where Mewtwo's sign is located.
After the initial descent, you pass several Dittos disguised as Bulbasaurs. After this, you enter a large cavern where the Jigglypuffs and Koffings battle. The third Jigglypuff and Koffing appear from a dark area to the left. You can see it clearly as you approach the area with Articuno. In this dark area, a strange constellation of lights can be seen. This is actually the Pokémon Sign. Aim for the largest light and take a photo of all of them. You won't be able to see Mewtwo there yet, but don't worry.You can't see Mewtwo yet, but that is supposed to happen.
Finish the stage normally and get your photos uploaded to Prof. Oak. If you got it right, the ghostly image of Mewtwo should be visible on the shot.And there he is. Spooky or what?