First, capture a nincada. Make it envolve at level 20. Make sure when it envolves you must have one empty space in your party. After when your nincada involves into a ninjask, look at your party pokemons. You will see that Shedninja is sitting on the empty spot of your party. It will have the sames moves as your Ninjask. Shedninja also only has one hp, but is only affected with moves that are super effective against it.