"The Nirvana formula" indicates that the author believes Nirvana had an obvious pattern or approach to structuring their songs: begin more quietly w/a descending series of notes, then build in energy/speed/volume to a loud & emotional chorus.
"The Nirvana formula personified" basically means that the author thinks "Heart-Shaped Box" is a good example of that formula, and that the song itself takes on human characteristics of being emotional and energetic. This is a bit more interesting than saying "it's a good example of the Nirvana formula."
It begins more restrained & not so loud/intense.
It opens with a "descending riff" meaning it starts from a higher tone/note and moves through a short/repeated/identifiable series of notes to a lower one.
"Building intensity" means that as the song progresses, it gets louder, faster, heavier, more "intense".
A "cascade" is like a waterfall, or something continuously overflowing/falling. Upon reaching the chorus, the passion with which Kurt Cobain sings/plays, and the music itself becomes overflowing & unrestrained. Contrast the screaming vocals & distorted guitars of the chorus with the relatively calm lyrics & non-distorted guitar of the verses, and you see the "formula" at work.