First of all, you will need to be playing Ladder mode. It's best to set the difficulty to beginner.

You will need to wait for The Pit stage. Once The Pit stage comes up, you will need to get either a witch or Santa Claus fly by the moon.


If you are lucky to get the Witch or Santa fly by, you will need to get a double flawless victory (This is why it's best to set the game difficulty to beginner. If The Pit stage does not come up by the time you reach the 3rd opponent, start over again, because the AI difficulty will increase when you get to the 4th opponent)


When it's time to finish off your opponent, you will need to perform a Stage Fatality


Once you meet all those requirements you will fight classic Reptile.