var fs = require('fs');

var page=this;

var result = page.evaluate(function() {

    var out;
    $.ajax({
        'async' : false,
        'url' : 'fullurltodownload.csv',
        'success' : function(data, status, xhr) {
            out = data;
        }
    });
    return out;

});

fs.write('mydownloadedfile.csv', result);
[download]

That is essentially what i suggested in first reply, make a request/get/post... then access the download with content method https://metacpan.org/pod/WWW::Mechanize::PhantomJS#mech-content-options

Thank you very much for the direction.