=IMPORTXML(URL,CONCATENATE("//*[local-name() = '",*ELEMENTNAME*,"']"))

//*/[local-name() = 'filing-date']
//*/[local-name() = 'size']
etc.

//*[local-name() = 'filing-date'] | //*[local-name() = 'filing-type'] | //*[local-name() = 'filing-href'] | //*[local-name() = 'form-name'] | //*[local-name() = 'size'] | //*[local-name() = 'amend']

function parseFeedXml(url) {
   var result = [];
   var xml = UrlFetchApp.fetch(url).getContentText();
   var document = XmlService.parse(xml);
   var atom = XmlService.getNamespace('http://www.w3.org/2005/Atom');
   var feed = document.getRootElement();
   var entry = feed.getChildren('entry',atom);
   for (var i = 0; i < entry.length; i++) {
     var content = entry[i].getChildren('content',atom);
     var contentElement = '';
     var resultRow = [];
     for (var j = 0; j < content.length; j++) {
       contentElement = content[j];
       evaluateText(contentElement.getChild('filing-date',atom), resultRow);
       evaluateText(contentElement.getChild('filing-type',atom), resultRow);
       evaluateText(contentElement.getChild('filing-href',atom), resultRow);
       evaluateText(contentElement.getChild('form-name',atom), resultRow);
       evaluateText(contentElement.getChild('size',atom), resultRow);
       evaluateText(contentElement.getChild('amend',atom), resultRow);
       result.push(resultRow);
     }
   }
   return result;
}

function evaluateText(element, resultRow) {
  if (element) {
    resultRow.push(element.getText());
  } else {
    resultRow.push('');
  }
}