You first need two NDS's
Say you want to clone mewtwo. Have your friend go and catch a Pokemon they don't need(It will be lost) something like a pidgey level 3.
Bring mew two,have your friend bring the useless pokemon.
Go to the union room and DON'T SAVE!!!!!!!!!!!!!!!!!!!!!!
trade your mewtwo to your friend's pidgey.
Have your friend save. YOU,turn off your power without saving.
Turn back on your power and go catch a useless pokemon(or a few).
with your useless pokemon,go to the union room you can save this time.
trade your useless pokemon with your friend,YOU,save but your friend DOESN'T.
have your friend turn off the power with out saving but you save.
Keep on trading until you have as many as you want.
REMEMBER,your friend never saves but you always do.
the last time you trade and both of you save,so your friend doesn't get your special mew two.
Now you can trade some of them with wi-fi,to get extremely good or rare pokemon.