Doing it in a practice match is only a quick way though, the jp wiki mentions another slower way, just take a single ship fleet to 1-1 and return to base after the first battle and keep doing that until it sparkles.
Name:
Anonymous2013-09-01 15:34
11) 不思議の大魔境: "大魔境" is translated as Pandemonium in the game, so it is "Mystery Pandemonium".
It is really hard. It becomes harder latter, when you can't kill high defense monsters like Kaguya. I couldn't beat it yet.