From the Super Mario Wiki
The Goomba Storm is a Special Attack Bowser uses in Mario & Luigi: Bowser's Inside Story. When using this, Bowser gets a Goomba Ball in his hand. After throwing the ball, Goombas charge at the chosen enemy. For extra damage, Bowser can launch a fireball at them. After the non-burning goombas have attacked, the burning ones fall over the foe. This moves costs 6 SP to use. To get an excellent, 18 Goombas have to be burned.
To use this move, Bowser has to free the Goombas in Dimble Wood first. It is the only required Enemy Attack as Bowser can just walk past the other military troops, except the Koopa Troopas. In the game, Bowser has to smash a wall to continue, also opening up the location of the Koopas to Bowser. He then has to continue, directly passing by the Koopas, who shout out pleads of mercy if not freed.
Bowser uses this attack during the second battle against him, as well as his Bowser X counterpart. Mario and Luigi can jump to avoid the non-burning goombas and then hammer the burning ones back at Bowser.
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