Well, this phrase is to be interpreted with how you believe it would be. Let's break it down. Let's start with the beginning of our sentence; "Who with a single thwart." What does the word "thwart" mean. Thwart: (verb) to prevent (someone) from accomplishing something. So, we have this so far; "Who with a single prevented blow."
Onto the next part, "Blow cleft asunder a brace"
Lets dive into the words "cleft" and "asunder." Cleft: (adjective) split, divided, in two. Asunder: (adverb) into pieces, apart
All together we now have; "Who with a single prevented blow split into pieces under a brace."
Lastly, we will define "of hulking blustering giants."
Lets break down this one more. When I see the word "hulking" I think of the Incredible Hulk. So let's dive into that word, and the word beside it.
The inability to keep a secret. Actually it implies a woman can't keep a secret, but then who can. The secret she can't keep...her husband had the ears of a donkey.