From the Super Mario Wiki
Thunder Bros. is one of Luigi's Bros. Attack in the game Mario & Luigi: Superstar Saga, based off of Luigi's Thunderhand move in the game. The move costs six BP on Modes 1 and 2, and four BP on Mode 3. Luigi will start a ball of electricity using his Thunderhand move. When fully charged, pressing the results in Luigi jumping on Mario. Another press on at this time will send Luigi flying into the air, and one more hit will allow Luigi to cast a thunderbolt below him, causing all enemies on the ground some damage. The attack, if performed correctly, always lowers the enemies' defense. The attack can also hit airborne enemies if the second command failed.
Once the attack has been successfully executed eighteen times, the Advanced version is learned. In the Advanced version, Luigi damages only one enemy with more damage. To execute it, the player should press after the initial charge. Before Luigi lands on Mario, pressing the makes Mario step backwards. again to pound Luigi into the ground, and again when Luigi's underneath his target. He'll attack with an electrifying uppercut that can even hit airborne enemies. The Advanced version, when performed correctly, will also lower the enemy's attack power.
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