Finally, after months of wait, someone uploaded the Raildun PSP game DLC walkthrough.
WTF, 13 minutes only?! ALL THIS WAIT FOR A 13 MINUTES!?
Sadly the guy has tons of "unnecessary" comments all the way, but at least, the walkthrough..
http://www.nicovideo.jp/watch/sm19109688
EDIT: mediafire link if nico is not working for you
http://www.mediafire.com/?pm5x9kh24k9h9qd