^Law has one of the most powerful devil fruits we have seen in the series. He can control almost anything in his 'rooms'...as so long as he can surpass their Haki. The only reason Law has suffered so far has been because his heart was being controlled by Vergo. Once he had his heart back, and he could equal or surpass Vergo's Haki, it is not surprising that he could defeat Vergo (though I agree the entire mountain being 'cut' was also surprising.
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