不堪一击 = to be unable to withstand a single blow / to collapse at the first blow http://www.mdbg.net/chindict/chindict.php?page=worddict&wdrst=0&wdqb=%E4%B8%8D%E5%A0%AA%E4%B8%80%E5%87%BB

not withstand a single blow

Collapsing at the first blow