From the Super Mario Wiki
Elite Goombules are a stronger variation of Goombule fought only once inside Bowser's body in Mario & Luigi: Bowser's Inside Story. They can be encountered in the Trash Pit. Like their weaker counterpart, Elite Goombules have an egg yolk shaped body with a yellow core in the center, but they wear Buzzy Beetle shells that deflect jump attacks and counterattacks as helmets. To defeat one, the bros. have to use their newly learned Green Shell attack obtained by collecting all ten Attack Parts from the Trash Pit. Unlike their regular counterpart, Elite Goombules are blue, while regular Goombules are red.
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