From the Super Mario Wiki
Like a regular Milde, Marching Milde does not attack, instead simply walking left and right inside its large chamber, hurting the Yellow Yoshi if he touches it from any other side than from above. As is the case with Large Mildes, jumping on Marching Milde simply causes the Yoshi to bounce off without doing any damage. To defeat this boss, the player must instead Ground Pound on top of Marching Milde to make it split apart into two smaller versions of itself. Employing the same strategy against the smaller Marching Mildes makes them split into Large Mildes, which in turn split into regular Mildes upon being Ground Pounded. Once every Milde has been destroyed or eaten, the boss is defeated.
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