Swizz tha Monster, he could star in a horror movie
Swizz Beats, is also known by the alias of as Swizz Beatz The Monsta.
So a monster is a typical horror movie character that scares the people in the movie.
So Cassidy is just using play on words here, most likely desrcribing Swizz Beatz production skills, as Swizz has produced a bunch of tracks for Cassidy.
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