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[edit] Empress Bulbax (Hole of Beasts)

Not very hard at all. You will approach from behind, while it is still sleeping. If any Pikmin attack it, the fight will begin. Hit it with Purples, because they are more aggressive. After inflicting enough damage, it will try to shake your Pikmin off. If it does so, your Pikmin will be flung towards it, and it will roll over your Pikmin. Simply whistle your Pikmin off it.