From the Super Mario Wiki
After they spot Wario, they will shoot a laser from their eyeball-like undersides and slowly turn over until they are upside down. This is the only time when they are vulnerable. Wario must Ground Pound their soft undersides before they flip back over. It takes one Ground Pound to stun a Laser Jigglefish, and a Mad Move such as a Piledriver to defeat it.
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