To unlock Jigglypuff you must've first completed Subspace Emissary and then load it and go to "The Swamp". When going to "The Swamp" choose any characters and any difficulty, won't make a difference in unlocking Jigglypuff. Then keep going in the stage and skip the first door you see. The next door you see you have no choice but to enter and you'll fight Dark Diddy Kong, he's not too hard nor too easy, and after beating him then you will go back to stage in different area and keep going until next door which you enter but this time Jigglypuff will be waiting. Defeat Jigglypuff then you can unlock him!