From the Super Mario Wiki
Guardian Shroobs are white Shroobs with blue spots. They are found in the Shroob Mother Ship during the events of Mario & Luigi: Partners in Time. They are stronger and have faster attacks than normal Shroobs, and often attack in groups. However, there are only two Guardian Shroobs encountered in the the overworld and only a total of nine appearing in the whole game. After the four Mario Bros. fall out of the airship, Guardian Shroobs cannot be battled again.
In battle, a Guardian Shroob can call another Guardian Shroob to approach the Mario brothers and fire two quick energy blasts from their ray guns, one for each Mario brother. To dodge this attack, the Mario brothers can simply jump over the energy blasts. They can also attack by themselves, just like an ordinary Shroob, though the energy blast is way faster than a normal Shroob's.
The Shroobs fought in Mario & Luigi: Bowser's Inside Story share a similar attack pattern.
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