In the winter world there is a dungeon (I can't remember the name but it is the first one after the bridge.) Take one pikmin with you down there and work your way to the 8th level. Use the multicolored flowers first and bring up your pikmin supply up to about 18. Then throw those pikmin into the three purple flowers. Quit the dungeon and do the same thing again (you can to the dungeon as many times at you want over the course of day, so you can rack up the pikmin very quickly in that span of time.)
