Criticism is a study by which men grow important and formidable at very small expense. He whom nature has made weak, and idleness keeps ignorant, may yet support his vanity by the name of a critic. Johnson, Samuel
To criticize (destroy) is easier than to create?
I think it's saying that Criticism is an "art" in which a man is either brought down and the one who is criticizing is "growing important" at a "very small expense" (meaning, only because of little harsh words, they are lifting themselves up). At the same time, the people who are being brought down, is the people "whom nature has made weak" (because of all the criticism), and so he becomes bitter and is in vain, and thus, "supports his vanity" by also being a critic himself. So this quote is saying that Criticism is a study of a cycle of harsh words in life, I'm guessing :).
oh i didn't mean "either brought down" i just meant "brought down" :) sorry, my bad :)