This should be included as a tips.
For those that are getting Failed to retrieve ... during patching, download the failed file straight form Gravity's ftp:
ftp://ragnarok.nowcdn.co.kr:20021/Patch/ <- link might change over time if this need updating let me know
Put the downloaded file in your RO folder, at the same level as the rsu patcher then patch again. When the patcher reaches the file it will not fail anymore. Repeat for other failed files.