Question

“who with a single thwart blow cleft asunder a brace of hulking blustering giants”
What does this mean?

Answer 1

Is it a riddle or a quote?

Answer 2

Well lets start by defining some of the words
thwart - prevent someone from accomplishing something
cleft - split, divided, or partially divided into two
asunder - apart; divide into pieces
hulking - a person or object that's large, heavy, or clumsy
blustering - talking in a loud, aggressive,

Answer 3

Now we can fill on the quote like this:
Who With A Single (prevention) Blow (cleft and asunder both mean about the same thing "split or divide") A (pair) of (large, heavy) (obnoxious, agressive) Giants