From the Super Mario Wiki
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They attack Donkey Kong by curling into a ball and rolling towards him. Once they stop, Donkey Kong can use the Sound Wave Attack, causing him to engage a fallen Goal Mondo. He first kicks it into the air, and then, after hitting it back into the air a number of times, punches it into submission. Once defeated, they leave behind five beats.
They first appear in Massive Canyon, where one resides at the end of the level. It takes Donkey Kong five hits to defeat it. One later appears at the end of Ancient Foundry. It takes seven hits to defeat. A third Goal Mondo appears in Magma Coliseum.