First, you need 50 red Pikmin for this, when you get to the circular area that he is in, dismiss 30 Pikmin, but keep them away from the mushroom, but not too far. Get the other 20 to attack it's legs (if it looks like it's sucking in air, get your Pikmin away from it!)hopefully, it will tip over.When it does, throw the Pikmin on it, as fast as you can! When it gets upright get your Pikmin away, if they don't, the mushroom will spread a spore and turn them against you, get your other 30 Pikmin to battle the evi ones, repeat until it is dead. Also the top is the missing part, the other half is a lot of pellets.