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Jigglypuff has a smash that allows it to grow big, but only for a short time. This guide will tell you how to make jiggly puff big for ever. you can only do this on Bridge of Eldin and Pirate Ship, and smash balls need to be on.
For Bridge Eldin, break the smash ball with Jigglypuff, then wait until the goblin guy to drop the bomb. let the bridge explode. When the bridge starts to regenerate, go right in the gap of the bridge and activate the smash. Jigglypuff should start to grow in the same place the bridge is regenerating. Mid-growth, jiggly will stop growing and get knocked out on the fully regenerated bridge, except now jiggly is huge. Jiggly will move very slowly and it's attacks will still do the same damage.
For Pirate Ship, grab the smash ball, then wait for the rock island. The ship should be tilted upwards. go under the tilted ship, and activate the smash when the ship starts to tilt down and away from the island. Jiggly will stop growing mid-growth, and be big.
For more fun, if you have a friend, make them play as Yoshi. Make Yoshi use his B attack, and make him eat giant Jiggly puff. When Jiggly pops out as an egg, Jiggly will be HUGE.