You need at least 1 of any 2 balls for this to work, during your first pokemon's turn, pick the pokeball that you want to use and throw it at the shadow pokemon that you want, on your second pokemon's turn switch the pokeball that you used with another type and on your next turn you will notice that the one that you usedis still there and this glitch will work on masterballs too, so when you get the masterball you won't have to worry about killing any shadow pokemon.