From the Super Mario Wiki
Marching Milde is a giant Milde appearing in Super Mario World 2: Yoshi's Island and its remake, Yoshi's Island: Super Mario Advance 3. She is the mid-boss of World 4, encountered at the end of Marching Milde's Fort by Yellow Yoshi.
Like a regular Milde, Marching Milde does not attack, instead simply walking left and right inside her large chamber, hurting Yoshi if he touches her from any other side than from above. As is the case with Puchipuchi L, 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 her split apart into two smaller versions of herself, which are large versions of Puchipuchi L. Employing the same strategy against the large Puchipuchi L makes them split into regular-sized Puchipuchi L, which in turn split into Mildes upon being Ground Pounded. Once every Milde has been destroyed or eaten, the boss is defeated; this makes Marching Milde the only boss in the game that can be defeated by eating them.
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